xenial (7) cmake-developer.7.gz

Provided by: cmake-data_3.5.1-1ubuntu3_all bug

NAME

       cmake-developer - CMake Developer Reference

INTRODUCTION

       This manual is intended for reference by developers modifying the CMake source tree itself.

PERMITTED C++ SUBSET

       CMake  is required to build with ancient C++ compilers and standard library implementations.  Some common
       C++ constructs may not be used in CMake in order to build with such toolchains.

   std::auto_ptr
       Some implementations have a std::auto_ptr which can not be used  as  a  return  value  from  a  function.
       std::auto_ptr may not be used. Use cmsys::auto_ptr instead.

   size_t
       Various implementations have differing implementation of size_t.  When assigning the result of .size() on
       a container for example, the result should be assigned to size_t not  to  std::size_t,  unsigned  int  or
       similar types.

ADDING COMPILE FEATURES

       CMake  reports  an  error if a compiler whose features are known does not report support for a particular
       requested feature.  A compiler is considered to have known features if it reports support  for  at  least
       one feature.

       When adding a new compile feature to CMake, it is therefore necessary to list support for the feature for
       all CompilerIds which already have one or more feature supported, if the new feature is available for any
       version of the compiler.

       When  adding  the first supported feature to a particular CompilerId, it is necessary to list support for
       all features known to cmake (See CMAKE_C_COMPILE_FEATURES and CMAKE_CXX_COMPILE_FEATURES as appropriate),
       where  available  for the compiler.  Ensure that the CMAKE_<LANG>_STANDARD_DEFAULT is set to the computed
       internal variable CMAKE_<LANG>_STANDARD_COMPUTED_DEFAULT for compiler versions which should be supported.

       It is sensible to record the features for the most recent version of a particular CompilerId  first,  and
       then  work backwards.  It is sensible to try to create a continuous range of versions of feature releases
       of the compiler.  Gaps in the range indicate incorrect features recorded for intermediate releases.

       Generally, features are made available  for  a  particular  version  if  the  compiler  vendor  documents
       availability of the feature with that version.  Note that sometimes partially implemented features appear
       to be functional in previous  releases  (such  as  cxx_constexpr  in  GNU  4.6,  though  availability  is
       documented  in  GNU  4.7),  and  sometimes  compiler  vendors  document  availability of features, though
       supporting  infrastructure  is  not  available  (such  as  __has_feature(cxx_generic_lambdas)  indicating
       non-availability  in  Clang  3.4, though it is documented as available, and fixed in Clang 3.5).  Similar
       cases for other compilers and versions need to be investigated when extending CMake to support them.

       When a vendor releases a new version of a known compiler which supports a previously unsupported feature,
       and  there  are  already  known  features for that compiler, the feature should be listed as supported in
       CMake for that version of the compiler as soon as reasonably possible.

       Standard-specific/compiler-specific variables  such  CMAKE_CXX98_COMPILE_FEATURES  are  deliberately  not
       documented.  They only exist for the compiler-specific implementation of adding the -std compile flag for
       compilers which need that.

HELP

       The Help directory contains CMake help manual source files.  They are written using the  reStructuredText
       markup syntax and processed by Sphinx to generate the CMake help manuals.

   Markup Constructs
       In  addition  to  using Sphinx to generate the CMake help manuals, we also use a C++-implemented document
       processor to print documents for the --help-*  command-line  help  options.   It  supports  a  subset  of
       reStructuredText markup.  When authoring or modifying documents, please verify that the command-line help
       looks good in addition to the Sphinx-generated html and man pages.

       The command-line help processor supports the following constructs defined  by  reStructuredText,  Sphinx,
       and a CMake extension to Sphinx.

       CMake Domain directives
              Directives  defined  in  the  CMake Domain for defining CMake documentation objects are printed in
              command-line help output as if the lines were normal paragraph text with interpretation.

       CMake Domain interpreted text roles
              Interpreted text roles defined in the  CMake  Domain  for  cross-referencing  CMake  documentation
              objects  are  replaced  by  their  link text in command-line help output.  Other roles are printed
              literally and not processed.

       code-block directive
              Add a literal code block without interpretation.  The command-line help processor prints the block
              content without the leading directive line and with common indentation replaced by one space.

       include directive
              Include  another  document  source  file.   The  command-line  help  processor prints the included
              document inline with the referencing document.

       literal block after ::
              A paragraph ending in :: followed by a blank line treats the following indented block  as  literal
              text  without  interpretation.  The command-line help processor prints the :: literally and prints
              the block content with common indentation replaced by one space.

       note directive
              Call out a side note.  The command-line help processor prints the block content as  if  the  lines
              were normal paragraph text with interpretation.

       parsed-literal directive
              Add  a literal block with markup interpretation.  The command-line help processor prints the block
              content without the leading directive line and with common indentation replaced by one space.

       productionlist directive
              Render context-free grammar productions.  The command-line help processor prints the block content
              as if the lines were normal paragraph text with interpretation.

       replace directive
              Define  a  |substitution|  replacement.   The  command-line help processor requires a substitution
              replacement to be defined before it is referenced.

       |substitution| reference
              Reference  a  substitution  replacement  previously  defined  by  the  replace   directive.    The
              command-line help processor performs the substitution and replaces all newlines in the replacement
              text with spaces.

       toctree directive
              Include other document sources in the Table-of-Contents  document  tree.   The  command-line  help
              processor prints the referenced documents inline as part of the referencing document.

       Inline  markup  constructs  not  listed  above are printed literally in the command-line help output.  We
       prefer to use inline markup constructs that look correct in source form, so avoid  use  of  \-escapes  in
       favor of inline literals when possible.

       Explicit  markup  blocks  not matching directives listed above are removed from command-line help output.
       Do not use them, except for plain .. comments that are removed by Sphinx too.

       Note that nested indentation of blocks is not recognized by the command-line help processor.  Therefore:

       • Explicit markup blocks are recognized only when not indented inside other blocks.

       • Literal blocks after paragraphs ending in :: but not at the  top  indentation  level  may  consume  all
         indented lines following them.

       Try to avoid these cases in practice.

   CMake Domain
       CMake  adds  a  Sphinx  Domain called cmake, also called the "CMake Domain".  It defines several "object"
       types for CMake documentation:

       command
              A CMake language command.

       generator
              A CMake native build system generator.  See the cmake(1) command-line tool's -G option.

       manual A CMake manual page, like this cmake-developer(7) manual.

       module A CMake module.  See the cmake-modules(7) manual and the include() command.

       policy A CMake policy.  See the cmake-policies(7) manual and the cmake_policy() command.

       prop_cache, prop_dir, prop_gbl, prop_sf, prop_inst, prop_test, prop_tgt
              A CMake cache,  directory,  global,  source  file,  installed  file,  test,  or  target  property,
              respectively.  See the cmake-properties(7) manual and the set_property() command.

       variable
              A CMake language variable.  See the cmake-variables(7) manual and the set() command.

       Documentation  objects  in  the CMake Domain come from two sources.  First, the CMake extension to Sphinx
       transforms every document named with the form Help/<type>/<file-name>.rst to a domain  object  with  type
       <type>.  The object name is extracted from the document title, which is expected to be of the form:

          <object-name>
          -------------

       and  to appear at or near the top of the .rst file before any other lines starting in a letter, digit, or
       <.  If no such title appears literally in the .rst file, the object name is the <file-name>.  If a  title
       does  appear,  it  is  expected  that  <file-name>  is equal to <object-name> with any < and > characters
       removed.

       Second, the CMake Domain provides directives to define objects inside other documents:

          .. command:: <command-name>

           This indented block documents <command-name>.

          .. variable:: <variable-name>

           This indented block documents <variable-name>.

       Object types for which no directive is available must be defined using the first approach above.

   Cross-References
       Sphinx uses reStructuredText interpreted text roles to provide cross-reference syntax.  The CMake  Domain
       provides  for  each domain object type a role of the same name to cross-reference it.  CMake Domain roles
       are inline markup of the forms:

          :type:`name`
          :type:`text <name>`

       where type is the domain object type and name is the domain object name.  In the first form the link text
       will be name (or name() if the type is command) and in the second form the link text will be the explicit
       text.  For example, the code:

          * The :command:`list` command.
          * The :command:`list(APPEND)` sub-command.
          * The :command:`list() command <list>`.
          * The :command:`list(APPEND) sub-command <list>`.
          * The :variable:`CMAKE_VERSION` variable.
          * The :prop_tgt:`OUTPUT_NAME_<CONFIG>` target property.

       produces:

       • The list() command.

       • The list(APPEND) sub-command.

       • The list() command.

       • The list(APPEND) sub-command.

       • The CMAKE_VERSION variable.

       • The OUTPUT_NAME_<CONFIG> target property.

       Note that CMake Domain roles differ from Sphinx and reStructuredText convention in that  the  form  a<b>,
       without a space preceding <, is interpreted as a name instead of link text with an explicit target.  This
       is necessary because we use <placeholders> frequently in object  names  like  OUTPUT_NAME_<CONFIG>.   The
       form a <b>, with a space preceding <, is still interpreted as a link text with an explicit target.

   Style
   Style: Section Headers
       When marking section titles, make the section decoration line as long as the title text.  Use only a line
       below the title, not above. For example:

          Title Text
          ----------

       Capitalize the first letter of each non-minor word in the title.

       The section header underline character hierarchy is

       • #: Manual group (part) in the master document

       • *: Manual (chapter) title

       • =: Section within a manual

       • -: Subsection or CMake Domain object document title

       • ^: Subsubsection or CMake Domain object document section

       • ": Paragraph or CMake Domain object document subsection

   Style: Whitespace
       Use two spaces for indentation.  Use two spaces between sentences in prose.

   Style: Line Length
       Prefer to restrict the width of lines to 75-80 columns.  This is not a hard restriction, but writing  new
       paragraphs wrapped at 75 columns allows space for adding minor content without significant re-wrapping of
       content.

   Style: Prose
       Use American English spellings in prose.

   Style: Starting Literal Blocks
       Prefer to mark the start of literal blocks with :: at the end of the preceding paragraph. In cases  where
       the following block gets a code-block marker, put a single : at the end of the preceding paragraph.

   Style: CMake Command Signatures
       Command signatures should be marked up as plain literal blocks, not as cmake code-blocks.

       Signatures are separated from preceding content by a section header.  That is, use:

          ... preceding paragraph.

          Normal Libraries
          ^^^^^^^^^^^^^^^^

          ::

            add_library(<lib> ...)

          This signature is used for ...

       Signatures  of commands should wrap optional parts with square brackets, and should mark list of optional
       arguments with an ellipsis (...).  Elements of the signature which are specified by the  user  should  be
       specified with angle brackets, and may be referred to in prose using inline-literal syntax.

   Style: Boolean Constants
       Use   "OFF"   and   "ON"   for   boolean   values   which   can   be   modified  by  the  user,  such  as
       POSITION_INDEPENDENT_CODE. Such properties may be "enabled" and "disabled". Use "True"  and  "False"  for
       inherent values which can't be modified after being set, such as the IMPORTED property of a build target.

   Style: Inline Literals
       Mark  up  references to keywords in signatures, file names, and other technical terms with inline-literal
       syntax, for example:

          If ``WIN32`` is used with :command:`add_executable`, the
          :prop_tgt:`WIN32_EXECUTABLE` target property is enabled. That command
          creates the file ``<name>.exe`` on Windows.

   Style: Cross-References
       Mark up linkable references as links, including repeats.  An alternative, which is used by  wikipedia  (‐
       http://en.wikipedia.org/wiki/WP:REPEATLINK),  is to link to a reference only once per article. That style
       is not used in CMake documentation.

   Style: Referencing CMake Concepts
       If referring to a concept which corresponds to a property, and that concept is described in a  high-level
       manual, prefer to link to the manual section instead of the property. For example:

          This command creates an :ref:`Imported Target <Imported Targets>`.

       instead of:

          This command creates an :prop_tgt:`IMPORTED` target.

       The latter should be used only when referring specifically to the property.

       References to manual sections are not automatically created by creating a section, but code such as:

          .. _`Imported Targets`:

       creates  a  suitable  anchor.   Use  an  anchor name which matches the name of the corresponding section.
       Refer to the anchor using a cross-reference with specified text.

       Imported Targets need the IMPORTED term marked up with care in particular because the term may refer to a
       command keyword (IMPORTED), a target property (IMPORTED), or a concept (Imported Targets).

       Where a property, command or variable is related conceptually to others, by for example, being related to
       the buildsystem description, generator expressions or Qt, each relevant  property,  command  or  variable
       should  link  to  the primary manual, which provides high-level information.  Only particular information
       relating to the command should be in the documentation of the command.

   Style: Referencing CMake Domain Objects
       When referring to CMake Domain objects such as properties, variables, commands etc, prefer to link to the
       target object and follow that with the type of object it is.  For example:

          Set the :prop_tgt:`AUTOMOC` target property to ``ON``.

       Instead of

          Set the target property :prop_tgt:`AUTOMOC` to ``ON``.

       The policy directive is an exception, and the type us usually referred to before the link:

          If policy :prop_tgt:`CMP0022` is set to ``NEW`` the behavior is ...

       However,  markup  self-references  with  inline-literal syntax.  For example, within the add_executable()
       command documentation, use

          ``add_executable``

       not

          :command:`add_executable`

       which is used elsewhere.

MODULES

       The Modules directory contains CMake-language .cmake module files.

   Module Documentation
       To document CMake module Modules/<module-name>.cmake, modify Help/manual/cmake-modules.7.rst to reference
       the module in the toctree directive, in sorted order, as:

          /module/<module-name>

       Then add the module document file Help/module/<module-name>.rst containing just the line:

          .. cmake-module:: ../../Modules/<module-name>.cmake

       The  cmake-module  directive  will  scan  the module file to extract reStructuredText markup from comment
       blocks that start in .rst:.  Add to the top of Modules/<module-name>.cmake a Line Comment  block  of  the
       form:

          #.rst:
          # <module-name>
          # -------------
          #
          # <reStructuredText documentation of module>

       or a Bracket Comment of the form:

          #[[.rst:
          <module-name>
          -------------

          <reStructuredText documentation of module>
          #]]

       Any  number  of  = may be used in the opening and closing brackets as long as they match.  Content on the
       line containing the closing bracket is excluded if and only if the line starts in #.

       Additional such .rst: comments may appear anywhere in the module file.  All such comments must start with
       # in the first column.

       For example, a Modules/Findxxx.cmake module may contain:

          #.rst:
          # FindXxx
          # -------
          #
          # This is a cool module.
          # This module does really cool stuff.
          # It can do even more than you think.
          #
          # It even needs two paragraphs to tell you about it.
          # And it defines the following variables:
          #
          # * VAR_COOL: this is great isn't it?
          # * VAR_REALLY_COOL: cool right?

          <code>

          #[========================================[.rst:
          .. command:: xxx_do_something

           This command does something for Xxx::

            xxx_do_something(some arguments)
          #]========================================]
          macro(xxx_do_something)
            <code>
          endmacro()

       After  the top documentation block, leave a BLANK line, and then add a copyright and licence notice block
       like this one (change only the year range and name)

          #=============================================================================
          # Copyright 2009-2011 Your Name
          #
          # Distributed under the OSI-approved BSD License (the "License");
          # see accompanying file Copyright.txt for details.
          #
          # This software is distributed WITHOUT ANY WARRANTY; without even the
          # implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
          # See the License for more information.
          #=============================================================================
          # (To distribute this file outside of CMake, substitute the full
          #  License text for the above reference.)

       Test the documentation formatting by running cmake --help-module <module-name>, and also by enabling  the
       SPHINX_HTML  and  SPHINX_MAN  options  to  build  the  documentation.   Edit the comments until generated
       documentation looks satisfactory.  To have a .cmake file in this directory NOT show  up  in  the  modules
       documentation,     simply     leave     out    the    Help/module/<module-name>.rst    file    and    the
       Help/manual/cmake-modules.7.rst toctree entry.

   Find Modules
       A "find module" is a Modules/Find<package>.cmake file to be loaded by  the  find_package()  command  when
       invoked for <package>.

       The  primary  task  of  a  find  module  is  to determine whether a package exists on the system, set the
       <package>_FOUND variable to reflect this and provide any variables, macros and imported targets  required
       to use the package.  A find module is useful in cases where an upstream library does not provide a config
       file package.

       The traditional approach is to use variables for everything, including libraries and executables: see the
       Standard  Variable Names section below.  This is what most of the existing find modules provided by CMake
       do.

       The more modern approach is to behave as much like config file packages files as possible,  by  providing
       imported target.  This has the advantage of propagating Target Usage Requirements to consumers.

       In  either case (or even when providing both variables and imported targets), find modules should provide
       backwards compatibility with old versions that had the same name.

       A FindFoo.cmake module will typically be loaded by the command:

          find_package(Foo [major[.minor[.patch[.tweak]]]]
                       [EXACT] [QUIET] [REQUIRED]
                       [[COMPONENTS] [components...]]
                       [OPTIONAL_COMPONENTS components...]
                       [NO_POLICY_SCOPE])

       See the find_package() documentation for details on what variables are set for the find module.  Most  of
       these are dealt with by using FindPackageHandleStandardArgs.

       Briefly,  the module should only locate versions of the package compatible with the requested version, as
       described by the Foo_FIND_VERSION family of variables.  If Foo_FIND_QUIETLY is set  to  true,  it  should
       avoid  printing  messages,  including  anything  complaining  about  the  package  not  being  found.  If
       Foo_FIND_REQUIRED is set to true, the module should issue a FATAL_ERROR if the package cannot  be  found.
       If neither are set to true, it should print a non-fatal message if it cannot find the package.

       Packages  that  find  multiple  semi-independent  parts (like bundles of libraries) should search for the
       components listed in Foo_FIND_COMPONENTS if it is set , and only  set  Foo_FOUND  to  true  if  for  each
       searched-for  component  <c>  that  was  not  found,  Foo_FIND_REQUIRED_<c>  is  not  set  to  true.  The
       HANDLE_COMPONENTS argument of find_package_handle_standard_args() can be used to implement this.

       If Foo_FIND_COMPONENTS is not set, which modules are searched for and required is up to the find  module,
       but should be documented.

       For  internal  implementation,  it  is  a  generally  accepted  convention  that  variables starting with
       underscore are for temporary use only.

       Like all  modules,  find  modules  should  be  properly  documented.   To  add  a  module  to  the  CMake
       documentation, follow the steps in the Module Documentation section above.

   Standard Variable Names
       For a FindXxx.cmake module that takes the approach of setting variables (either instead of or in addition
       to creating imported targets), the following variable names should be  used  to  keep  things  consistent
       between  find  modules.   Note that all variables start with Xxx_ to make sure they do not interfere with
       other find modules; the same consideration applies to macros, functions and imported targets.

       Xxx_INCLUDE_DIRS
              The final set of include directories listed in one variable for use by client code.   This  should
              not be a cache entry.

       Xxx_LIBRARIES
              The  libraries  to link against to use Xxx. These should include full paths.  This should not be a
              cache entry.

       Xxx_DEFINITIONS
              Definitions to use when compiling code that uses Xxx. This really shouldn't include  options  such
              as -DHAS_JPEG that a client source-code file uses to decide whether to #include <jpeg.h>

       Xxx_EXECUTABLE
              Where to find the Xxx tool.

       Xxx_Yyy_EXECUTABLE
              Where to find the Yyy tool that comes with Xxx.

       Xxx_LIBRARY_DIRS
              Optionally,  the  final  set of library directories listed in one variable for use by client code.
              This should not be a cache entry.

       Xxx_ROOT_DIR
              Where to find the base directory of Xxx.

       Xxx_VERSION_Yy
              Expect Version Yy if true. Make sure at most one of these is ever true.

       Xxx_WRAP_Yy
              If False, do not try to use the relevant CMake wrapping command.

       Xxx_Yy_FOUND
              If False, optional Yy part of Xxx system is not available.

       Xxx_FOUND
              Set to false, or undefined, if we haven't found, or don't want to use Xxx.

       Xxx_NOT_FOUND_MESSAGE
              Should be set by config-files in the case that it has  set  Xxx_FOUND  to  FALSE.   The  contained
              message  will  be printed by the find_package() command and by find_package_handle_standard_args()
              to inform the user about the problem.

       Xxx_RUNTIME_LIBRARY_DIRS
              Optionally, the runtime library search path for use when running an executable  linked  to  shared
              libraries.   The list should be used by user code to create the PATH on windows or LD_LIBRARY_PATH
              on UNIX.  This should not be a cache entry.

       Xxx_VERSION
              The full version string of the package found, if any.  Note that  many  existing  modules  provide
              Xxx_VERSION_STRING instead.

       Xxx_VERSION_MAJOR
              The major version of the package found, if any.

       Xxx_VERSION_MINOR
              The minor version of the package found, if any.

       Xxx_VERSION_PATCH
              The patch version of the package found, if any.

       The following names should not usually be used in CMakeLists.txt files, but are typically cache variables
       for users to edit and control the behaviour of  find  modules  (like  entering  the  path  to  a  library
       manually)

       Xxx_LIBRARY
              The path of the Xxx library (as used with find_library(), for example).

       Xxx_Yy_LIBRARY
              The  path  of  the Yy library that is part of the Xxx system. It may or may not be required to use
              Xxx.

       Xxx_INCLUDE_DIR
              Where to find headers for using the Xxx library.

       Xxx_Yy_INCLUDE_DIR
              Where to find headers for using the Yy library of the Xxx system.

       To prevent users being overwhelmed with settings to configure, try to keep as many  options  as  possible
       out of the cache, leaving at least one option which can be used to disable use of the module, or locate a
       not-found library (e.g. Xxx_ROOT_DIR).  For the same reason, mark most cache options  as  advanced.   For
       packages  which  provide  both  debug and release binaries, it is common to create cache variables with a
       _LIBRARY_<CONFIG> suffix, such as Foo_LIBRARY_RELEASE and Foo_LIBRARY_DEBUG.

       While these are the standard variable names, you should provide backwards compatibility for any old names
       that were actually in use.  Make sure you comment them as deprecated, so that no-one starts using them.

   A Sample Find Module
       We will describe how to create a simple find module for a library Foo.

       The  first thing that is needed is documentation.  CMake's documentation system requires you to start the
       file with a documentation marker and the name of the module.   You  should  follow  this  with  a  simple
       statement of what the module does.

          #.rst:
          # FindFoo
          # -------
          #
          # Finds the Foo library
          #

       More  description  may be required for some packages.  If there are caveats or other details users of the
       module should be aware of, you can add further paragraphs below this.  Then you  need  to  document  what
       variables and imported targets are set by the module, such as

          # This will define the following variables::
          #
          #   Foo_FOUND    - True if the system has the Foo library
          #   Foo_VERSION  - The version of the Foo library which was found
          #
          # and the following imported targets::
          #
          #   Foo::Foo   - The Foo library

       If  the  package  provides  any  macros, they should be listed here, but can be documented where they are
       defined.  See the Module Documentation section above for more details.

       After the documentation, leave a blank line, and then add a copyright and licence notice block

          #=============================================================================
          # Copyright 2009-2011 Your Name
          #
          # Distributed under the OSI-approved BSD License (the "License");
          # see accompanying file Copyright.txt for details.
          #
          # This software is distributed WITHOUT ANY WARRANTY; without even the
          # implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
          # See the License for more information.
          #=============================================================================
          # (To distribute this file outside of CMake, substitute the full
          #  License text for the above reference.)

       Now the actual libraries and so on have to be found.  The code here will obviously vary  from  module  to
       module  (dealing  with  that,  after  all,  is the point of find modules), but there tends to be a common
       pattern for libraries.

       First, we try to use pkg-config to find the library.  Note that we cannot rely on this, as it may not  be
       available, but it provides a good starting point.

          find_package(PkgConfig)
          pkg_check_modules(PC_Foo QUIET Foo)

       This should define some variables starting PC_Foo_ that contain the information from the Foo.pc file.

       Now  we  need  to find the libraries and include files; we use the information from pkg-config to provide
       hints to CMake about where to look.

          find_path(Foo_INCLUDE_DIR
            NAMES foo.h
            PATHS ${PC_Foo_INCLUDE_DIRS}
            PATH_SUFFIXES Foo
          )
          find_library(Foo_LIBRARY
            NAMES foo
            PATHS ${PC_Foo_LIBRARY_DIRS}
          )

       If you have a good way of getting the version (from a  header  file,  for  example),  you  can  use  that
       information   to   set   Foo_VERSION   (although   note   that   find  modules  have  traditionally  used
       Foo_VERSION_STRING, so you may want to set  both).   Otherwise,  attempt  to  use  the  information  from
       pkg-config

          set(Foo_VERSION ${PC_Foo_VERSION})

       Now we can use FindPackageHandleStandardArgs to do most of the rest of the work for us

          include(FindPackageHandleStandardArgs)
          find_package_handle_standard_args(Foo
            FOUND_VAR Foo_FOUND
            REQUIRED_VARS
              Foo_LIBRARY
              Foo_INCLUDE_DIR
            VERSION_VAR Foo_VERSION
          )

       This  will  check  that the REQUIRED_VARS contain values (that do not end in -NOTFOUND) and set Foo_FOUND
       appropriately.  It will also cache those values.  If Foo_VERSION is  set,  and  a  required  version  was
       passed  to  find_package(),  it will check the requested version against the one in Foo_VERSION.  It will
       also print messages as appropriate; note that if the package was found, it will print the contents of the
       first required variable to indicate where it was found.

       At  this point, we have to provide a way for users of the find module to link to the library or libraries
       that were found.  There are two approaches,  as  discussed  in  the  Find  Modules  section  above.   The
       traditional variable approach looks like

          if(Foo_FOUND)
            set(Foo_LIBRARIES ${Foo_LIBRARY})
            set(Foo_INCLUDE_DIRS ${Foo_INCLUDE_DIR})
            set(Foo_DEFINITIONS ${PC_Foo_CFLAGS_OTHER})
          endif()

       If  more  than one library was found, all of them should be included in these variables (see the Standard
       Variable Names section for more information).

       When providing imported targets, these  should  be  namespaced  (hence  the  Foo::  prefix);  CMake  will
       recognize  that values passed to target_link_libraries() that contain :: in their name are supposed to be
       imported targets (rather than just library names), and will produce appropriate  diagnostic  messages  if
       that target does not exist (see policy CMP0028).

          if(Foo_FOUND AND NOT TARGET Foo::Foo)
            add_library(Foo::Foo UNKNOWN IMPORTED)
            set_target_properties(Foo::Foo PROPERTIES
              IMPORTED_LOCATION "${Foo_LIBRARY}"
              INTERFACE_COMPILE_OPTIONS "${PC_Foo_CFLAGS_OTHER}"
              INTERFACE_INCLUDE_DIRECTORIES "${Foo_INCLUDE_DIR}"
            )
          endif()

       One thing to note about this is that the INTERFACE_INCLUDE_DIRECTORIES and similar properties should only
       contain information  about  the  target  itself,  and  not  any  of  its  dependencies.   Instead,  those
       dependencies  should also be targets, and CMake should be told that they are dependencies of this target.
       CMake will then combine all the necessary information automatically.

       The type of the IMPORTED target created in the add_library() command can always be specified  as  UNKNOWN
       type.   This  simplifies  the  code in cases where static or shared variants may be found, and CMake will
       determine the type by inspecting the files.

       If the library is available with multiple configurations,  the  IMPORTED_CONFIGURATIONS  target  property
       should also be populated:

          if(Foo_FOUND)
            if (NOT TARGET Foo::Foo)
              add_library(Foo::Foo UNKNOWN IMPORTED)
            endif()
            if (Foo_LIBRARY_RELEASE)
              set_property(TARGET Foo::Foo APPEND PROPERTY
                IMPORTED_CONFIGURATIONS RELEASE
              )
              set_target_properties(Foo::Foo PROPERTIES
                IMPORTED_LOCATION_RELEASE "${Foo_LIBRARY_RELEASE}"
              )
            endif()
            if (Foo_LIBRARY_DEBUG)
              set_property(TARGET Foo::Foo APPEND PROPERTY
                IMPORTED_CONFIGURATIONS DEBUG
              )
              set_target_properties(Foo::Foo PROPERTIES
                IMPORTED_LOCATION_DEBUG "${Foo_LIBRARY_DEBUG}"
              )
            endif()
            set_target_properties(Foo::Foo PROPERTIES
              INTERFACE_COMPILE_OPTIONS "${PC_Foo_CFLAGS_OTHER}"
              INTERFACE_INCLUDE_DIRECTORIES "${Foo_INCLUDE_DIR}"
            )
          endif()

       The  RELEASE  variant  should  be listed first in the property so that that variant is chosen if the user
       uses a configuration which is not an exact match for any listed IMPORTED_CONFIGURATIONS.

       Most of the cache variables should be hidden in the ccmake interface unless the user explicitly  asks  to
       edit them.

          mark_as_advanced(
            Foo_INCLUDE_DIR
            Foo_LIBRARY
          )

       If  this  module  replaces  an  older  version, you should set compatibility variables to cause the least
       disruption possible.

          # compatibility variables
          set(Foo_VERSION_STRING ${Foo_VERSION})

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