Provided by: cmake-data_3.13.4-1build1_all bug

NAME

       cmake-packages - CMake Packages Reference

INTRODUCTION

       Packages  provide  dependency information to CMake based buildsystems.  Packages are found
       with the find_package() command.  The result of using find_package  is  either  a  set  of
       IMPORTED targets, or a set of variables corresponding to build-relevant information.

USING PACKAGES

       CMake  provides  direct  support  for  two  forms  of  packages,  Config-file Packages and
       Find-module Packages.  Indirect support for pkg-config packages is also provided  via  the
       FindPkgConfig module.  In all cases, the basic form of find_package() calls is the same:

          find_package(Qt4 4.7.0 REQUIRED) # CMake provides a Qt4 find-module
          find_package(Qt5Core 5.1.0 REQUIRED) # Qt provides a Qt5 package config file.
          find_package(LibXml2 REQUIRED) # Use pkg-config via the LibXml2 find-module

       In  cases where it is known that a package configuration file is provided by upstream, and
       only that should be used, the CONFIG keyword may be passed to find_package():

          find_package(Qt5Core 5.1.0 CONFIG REQUIRED)
          find_package(Qt5Gui 5.1.0 CONFIG)

       Similarly, the MODULE keyword says to use only a find-module:

          find_package(Qt4 4.7.0 MODULE REQUIRED)

       Specifying the type of package explicitly improves the error message shown to the user  if
       it is not found.

       Both  types  of packages also support specifying components of a package, either after the
       REQUIRED keyword:

          find_package(Qt5 5.1.0 CONFIG REQUIRED Widgets Xml Sql)

       or as a separate COMPONENTS list:

          find_package(Qt5 5.1.0 COMPONENTS Widgets Xml Sql)

       or as a separate OPTIONAL_COMPONENTS list:

          find_package(Qt5 5.1.0 COMPONENTS Widgets
                                 OPTIONAL_COMPONENTS Xml Sql
          )

       Handling of COMPONENTS and OPTIONAL_COMPONENTS is defined by the package.

       By  setting   the   CMAKE_DISABLE_FIND_PACKAGE_<PackageName>   variable   to   TRUE,   the
       <PackageName> package will not be searched, and will always be NOTFOUND.

   Config-file Packages
       A  config-file  package  is  a  set of files provided by upstreams for downstreams to use.
       CMake searches in a number of locations for package configuration files, as  described  in
       the  find_package()  documentation.  The most simple way for a CMake user to tell cmake(1)
       to search in a non-standard prefix for a package is to  set  the  CMAKE_PREFIX_PATH  cache
       variable.

       Config-file  packages  are  provided  by upstream vendors as part of development packages,
       that is, they belong with the  header  files  and  any  other  files  provided  to  assist
       downstreams in using the package.

       A  set  of  variables  which provide package status information are also set automatically
       when using a config-file package.  The <PackageName>_FOUND variable  is  set  to  true  or
       false,  depending  on whether the package was found.  The <PackageName>_DIR cache variable
       is set to the location of the package configuration file.

   Find-module Packages
       A find module is a file with a  set  of  rules  for  finding  the  required  pieces  of  a
       dependency, primarily header files and libraries.  Typically, a find module is needed when
       the upstream is not built with CMake, or is not CMake-aware enough to otherwise provide  a
       package  configuration  file.  Unlike a package configuration file, it is not shipped with
       upstream, but is used by downstream to find the files by guessing locations of files  with
       platform-specific hints.

       Unlike  the  case  of  an upstream-provided package configuration file, no single point of
       reference identifies the package as being found, so the  <PackageName>_FOUND  variable  is
       not  automatically  set by the find_package() command.  It can still be expected to be set
       by convention however and should be set by the author of the Find-module.  Similarly there
       is  no <PackageName>_DIR variable, but each of the artifacts such as library locations and
       header file locations provide a separate cache variable.

       See the cmake-developer(7) manual for more information about creating Find-module files.

PACKAGE LAYOUT

       A config-file package consists of a Package Configuration File and  optionally  a  Package
       Version File provided with the project distribution.

   Package Configuration File
       Consider a project Foo that installs the following files:

          <prefix>/include/foo-1.2/foo.h
          <prefix>/lib/foo-1.2/libfoo.a

       It may also provide a CMake package configuration file:

          <prefix>/lib/cmake/foo-1.2/FooConfig.cmake

       with content defining IMPORTED targets, or defining variables, such as:

          # ...
          # (compute PREFIX relative to file location)
          # ...
          set(Foo_INCLUDE_DIRS ${PREFIX}/include/foo-1.2)
          set(Foo_LIBRARIES ${PREFIX}/lib/foo-1.2/libfoo.a)

       If  another  project wishes to use Foo it need only to locate the FooConfig.cmake file and
       load it to get all the information it needs about package content  locations.   Since  the
       package  configuration  file  is provided by the package installation it already knows all
       the file locations.

       The find_package() command may be used to search for the package configuration file.  This
       command  constructs  a  set  of  installation  prefixes  and searches under each prefix in
       several locations.  Given the name Foo, it looks for  a  file  called  FooConfig.cmake  or
       foo-config.cmake.   The  full  set of locations is specified in the find_package() command
       documentation. One place it looks is:

          <prefix>/lib/cmake/Foo*/

       where Foo* is a  case-insensitive  globbing  expression.   In  our  example  the  globbing
       expression  will  match <prefix>/lib/cmake/foo-1.2 and the package configuration file will
       be found.

       Once found, a package configuration file is  immediately  loaded.   It,  together  with  a
       package version file, contains all the information the project needs to use the package.

   Package Version File
       When the find_package() command finds a candidate package configuration file it looks next
       to it for a version file. The version file is loaded to test whether the  package  version
       is   an  acceptable  match  for  the  version  requested.   If  the  version  file  claims
       compatibility the configuration file is accepted.  Otherwise it is ignored.

       The name of the package version file must match that of the package configuration file but
       has  either  -version  or  Version  appended to the name before the .cmake extension.  For
       example, the files:

          <prefix>/lib/cmake/foo-1.3/foo-config.cmake
          <prefix>/lib/cmake/foo-1.3/foo-config-version.cmake

       and:

          <prefix>/lib/cmake/bar-4.2/BarConfig.cmake
          <prefix>/lib/cmake/bar-4.2/BarConfigVersion.cmake

       are each pairs of package configuration files and corresponding package version files.

       When the find_package()  command  loads  a  version  file  it  first  sets  the  following
       variables:

       PACKAGE_FIND_NAME
              The <PackageName>

       PACKAGE_FIND_VERSION
              Full requested version string

       PACKAGE_FIND_VERSION_MAJOR
              Major version if requested, else 0

       PACKAGE_FIND_VERSION_MINOR
              Minor version if requested, else 0

       PACKAGE_FIND_VERSION_PATCH
              Patch version if requested, else 0

       PACKAGE_FIND_VERSION_TWEAK
              Tweak version if requested, else 0

       PACKAGE_FIND_VERSION_COUNT
              Number of version components, 0 to 4

       The  version  file  must use these variables to check whether it is compatible or an exact
       match for the requested version and set the following variables with results:

       PACKAGE_VERSION
              Full provided version string

       PACKAGE_VERSION_EXACT
              True if version is exact match

       PACKAGE_VERSION_COMPATIBLE
              True if version is compatible

       PACKAGE_VERSION_UNSUITABLE
              True if unsuitable as any version

       Version files are loaded in a nested scope so they are free to set any variables they wish
       as  part  of  their  computation.  The  find_package  command wipes out the scope when the
       version file has completed and it has checked the output variables. When the version  file
       claims  to  be an acceptable match for the requested version the find_package command sets
       the following variables for use by the project:

       <PackageName>_VERSION
              Full provided version string

       <PackageName>_VERSION_MAJOR
              Major version if provided, else 0

       <PackageName>_VERSION_MINOR
              Minor version if provided, else 0

       <PackageName>_VERSION_PATCH
              Patch version if provided, else 0

       <PackageName>_VERSION_TWEAK
              Tweak version if provided, else 0

       <PackageName>_VERSION_COUNT
              Number of version components, 0 to 4

       The  variables  report  the  version  of  the  package  that  was  actually  found.    The
       <PackageName> part of their name matches the argument given to the find_package() command.

CREATING PACKAGES

       Usually,  the  upstream  depends  on  CMake  itself  and can use some CMake facilities for
       creating the package files. Consider an upstream which provides a single shared library:

          project(UpstreamLib)

          set(CMAKE_INCLUDE_CURRENT_DIR ON)
          set(CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE ON)

          set(Upstream_VERSION 3.4.1)

          include(GenerateExportHeader)

          add_library(ClimbingStats SHARED climbingstats.cpp)
          generate_export_header(ClimbingStats)
          set_property(TARGET ClimbingStats PROPERTY VERSION ${Upstream_VERSION})
          set_property(TARGET ClimbingStats PROPERTY SOVERSION 3)
          set_property(TARGET ClimbingStats PROPERTY
            INTERFACE_ClimbingStats_MAJOR_VERSION 3)
          set_property(TARGET ClimbingStats APPEND PROPERTY
            COMPATIBLE_INTERFACE_STRING ClimbingStats_MAJOR_VERSION
          )

          install(TARGETS ClimbingStats EXPORT ClimbingStatsTargets
            LIBRARY DESTINATION lib
            ARCHIVE DESTINATION lib
            RUNTIME DESTINATION bin
            INCLUDES DESTINATION include
          )
          install(
            FILES
              climbingstats.h
              "${CMAKE_CURRENT_BINARY_DIR}/climbingstats_export.h"
            DESTINATION
              include
            COMPONENT
              Devel
          )

          include(CMakePackageConfigHelpers)
          write_basic_package_version_file(
            "${CMAKE_CURRENT_BINARY_DIR}/ClimbingStats/ClimbingStatsConfigVersion.cmake"
            VERSION ${Upstream_VERSION}
            COMPATIBILITY AnyNewerVersion
          )

          export(EXPORT ClimbingStatsTargets
            FILE "${CMAKE_CURRENT_BINARY_DIR}/ClimbingStats/ClimbingStatsTargets.cmake"
            NAMESPACE Upstream::
          )
          configure_file(cmake/ClimbingStatsConfig.cmake
            "${CMAKE_CURRENT_BINARY_DIR}/ClimbingStats/ClimbingStatsConfig.cmake"
            COPYONLY
          )

          set(ConfigPackageLocation lib/cmake/ClimbingStats)
          install(EXPORT ClimbingStatsTargets
            FILE
              ClimbingStatsTargets.cmake
            NAMESPACE
              Upstream::
            DESTINATION
              ${ConfigPackageLocation}
          )
          install(
            FILES
              cmake/ClimbingStatsConfig.cmake
              "${CMAKE_CURRENT_BINARY_DIR}/ClimbingStats/ClimbingStatsConfigVersion.cmake"
            DESTINATION
              ${ConfigPackageLocation}
            COMPONENT
              Devel
          )

       The  CMakePackageConfigHelpers  module  provides   a   macro   for   creating   a   simple
       ConfigVersion.cmake file.  This file sets the version of the package.  It is read by CMake
       when find_package() is called to determine the compatibility with the  requested  version,
       and      to     set     some     version-specific     variables     <PackageName>_VERSION,
       <PackageName>_VERSION_MAJOR, <PackageName>_VERSION_MINOR etc.  The install(EXPORT) command
       is  used  to export the targets in the ClimbingStatsTargets export-set, defined previously
       by the install(TARGETS) command. This  command  generates  the  ClimbingStatsTargets.cmake
       file  to contain IMPORTED targets, suitable for use by downstreams and arranges to install
       it to  lib/cmake/ClimbingStats.   The  generated  ClimbingStatsConfigVersion.cmake  and  a
       cmake/ClimbingStatsConfig.cmake  are  installed  to  the  same  location,  completing  the
       package.

       The generated IMPORTED targets have appropriate  properties  set  to  define  their  usage
       requirements,  such  as  INTERFACE_INCLUDE_DIRECTORIES,  INTERFACE_COMPILE_DEFINITIONS and
       other relevant built-in INTERFACE_ properties.   The  INTERFACE  variant  of  user-defined
       properties listed in COMPATIBLE_INTERFACE_STRING and other Compatible Interface Properties
       are  also  propagated  to  the  generated  IMPORTED   targets.    In   the   above   case,
       ClimbingStats_MAJOR_VERSION  is  defined  as  a  string which must be compatible among the
       dependencies of any depender.  By setting  this  custom  defined  user  property  in  this
       version  and  in  the  next  version of ClimbingStats, cmake(1) will issue a diagnostic if
       there is an attempt to use version 3 together with version  4.   Packages  can  choose  to
       employ  such  a  pattern  if  different  major  versions of the package are designed to be
       incompatible.

       A NAMESPACE with double-colons is specified when exporting the targets  for  installation.
       This  convention  of  double-colons gives CMake a hint that the name is an IMPORTED target
       when it is used by downstreams with the target_link_libraries() command.  This way,  CMake
       can issue a diagnostic if the package providing it has not yet been found.

       In  this  case,  when using install(TARGETS) the INCLUDES DESTINATION was specified.  This
       causes the IMPORTED targets to have their INTERFACE_INCLUDE_DIRECTORIES populated with the
       include  directory  in  the  CMAKE_INSTALL_PREFIX.   When  the  IMPORTED target is used by
       downstream, it automatically consumes the entries from that property.

   Creating a Package Configuration File
       In this case, the ClimbingStatsConfig.cmake file could be as simple as:

          include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")

       As this allows downstreams to use the IMPORTED targets.  If any macros should be  provided
       by  the ClimbingStats package, they should be in a separate file which is installed to the
       same location as the ClimbingStatsConfig.cmake file, and included from there.

       This can also be extended to cover dependencies:

          # ...
          add_library(ClimbingStats SHARED climbingstats.cpp)
          generate_export_header(ClimbingStats)

          find_package(Stats 2.6.4 REQUIRED)
          target_link_libraries(ClimbingStats PUBLIC Stats::Types)

       As the Stats::Types target is a PUBLIC dependency of ClimbingStats, downstreams must  also
       find  the Stats package and link to the Stats::Types library.  The Stats package should be
       found in the ClimbingStatsConfig.cmake file to ensure  this.   The  find_dependency  macro
       from  the  CMakeFindDependencyMacro  helps with this by propagating whether the package is
       REQUIRED, or QUIET etc.  All REQUIRED dependencies of a package should  be  found  in  the
       Config.cmake file:

          include(CMakeFindDependencyMacro)
          find_dependency(Stats 2.6.4)

          include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
          include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsMacros.cmake")

       The  find_dependency macro also sets ClimbingStats_FOUND to False if the dependency is not
       found, along with a diagnostic that the ClimbingStats package can not be used without  the
       Stats package.

       If  COMPONENTS  are  specified when the downstream uses find_package(), they are listed in
       the <PackageName>_FIND_COMPONENTS variable. If a  particular  component  is  non-optional,
       then the <PackageName>_FIND_REQUIRED_<comp> will be true. This can be tested with logic in
       the package configuration file:

          include(CMakeFindDependencyMacro)
          find_dependency(Stats 2.6.4)

          include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
          include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsMacros.cmake")

          set(_supported_components Plot Table)

          foreach(_comp ${ClimbingStats_FIND_COMPONENTS})
            if (NOT ";${_supported_components};" MATCHES _comp)
              set(ClimbingStats_FOUND False)
              set(ClimbingStats_NOT_FOUND_MESSAGE "Unsupported component: ${_comp}")
            endif()
            include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStats${_comp}Targets.cmake")
          endforeach()

       Here, the ClimbingStats_NOT_FOUND_MESSAGE is set to a diagnosis that the package could not
       be found because an invalid component was specified.  This message variable can be set for
       any case where the _FOUND variable is set to False, and will be displayed to the user.

   Creating a Package Configuration File for the Build Tree
       The export(EXPORT) command creates an IMPORTED targets definition file which  is  specific
       to  the  build-tree,  and  is not relocatable.  This can similarly be used with a suitable
       package configuration file and package version file to define a package for the build tree
       which  may  be  used  without installation.  Consumers of the build tree can simply ensure
       that the CMAKE_PREFIX_PATH contains the build directory, or set the  ClimbingStats_DIR  to
       <build_dir>/ClimbingStats in the cache.

   Creating Relocatable Packages
       A  relocatable package must not reference absolute paths of files on the machine where the
       package is built that will not exist on the machines where the package may be installed.

       Packages created by install(EXPORT) are designed to be relocatable, using  paths  relative
       to  the  location  of  the  package  itself.   When defining the interface of a target for
       EXPORT, keep in mind that the include directories should be specified  as  relative  paths
       which are relative to the CMAKE_INSTALL_PREFIX:

          target_include_directories(tgt INTERFACE
            # Wrong, not relocatable:
            $<INSTALL_INTERFACE:${CMAKE_INSTALL_PREFIX}/include/TgtName>
          )

          target_include_directories(tgt INTERFACE
            # Ok, relocatable:
            $<INSTALL_INTERFACE:include/TgtName>
          )

       The  $<INSTALL_PREFIX>  generator  expression may be used as a placeholder for the install
       prefix without resulting in a non-relocatable  package.   This  is  necessary  if  complex
       generator expressions are used:

          target_include_directories(tgt INTERFACE
            # Ok, relocatable:
            $<INSTALL_INTERFACE:$<$<CONFIG:Debug>:$<INSTALL_PREFIX>/include/TgtName>>
          )

       This  also  applies  to  paths  referencing external dependencies.  It is not advisable to
       populate any properties which may contain paths, such as INTERFACE_INCLUDE_DIRECTORIES and
       INTERFACE_LINK_LIBRARIES, with paths relevant to dependencies.  For example, this code may
       not work well for a relocatable package:

          target_link_libraries(ClimbingStats INTERFACE
            ${Foo_LIBRARIES} ${Bar_LIBRARIES}
            )
          target_include_directories(ClimbingStats INTERFACE
            "$<INSTALL_INTERFACE:${Foo_INCLUDE_DIRS};${Bar_INCLUDE_DIRS}>"
            )

       The referenced  variables  may  contain  the  absolute  paths  to  libraries  and  include
       directories  as found on the machine the package was made on.  This would create a package
       with hard-coded paths to dependencies and not suitable for relocation.

       Ideally such dependencies should be used through their  own  IMPORTED  targets  that  have
       their    own    IMPORTED_LOCATION    and    usage    requirement    properties   such   as
       INTERFACE_INCLUDE_DIRECTORIES populated appropriately.  Those imported targets may then be
       used with the target_link_libraries() command for ClimbingStats:

          target_link_libraries(ClimbingStats INTERFACE Foo::Foo Bar::Bar)

       With this approach the package references its external dependencies only through the names
       of IMPORTED targets.  When a consumer uses the installed package, the  consumer  will  run
       the appropriate find_package() commands (via the find_dependency macro described above) to
       find the dependencies and populate the imported targets with appropriate  paths  on  their
       own machine.

       Unfortunately  many modules shipped with CMake do not yet provide IMPORTED targets because
       their development pre-dated this approach.  This  may  improve  incrementally  over  time.
       Workarounds to create relocatable packages using such modules include:

       · When  building the package, specify each Foo_LIBRARY cache entry as just a library name,
         e.g. -DFoo_LIBRARY=foo.  This tells  the  corresponding  find  module  to  populate  the
         Foo_LIBRARIES  with  just  foo  to  ask  the linker to search for the library instead of
         hard-coding a path.

       · Or, after installing the package content but before creating  the  package  installation
         binary  for  redistribution,  manually  replace the absolute paths with placeholders for
         substitution by the installation tool when the package is installed.

PACKAGE REGISTRY

       CMake provides two central  locations  to  register  packages  that  have  been  built  or
       installed anywhere on a system:

       · User Package Registry

       · System Package Registry

       The  registries  are  especially  useful  to  help  projects find packages in non-standard
       install locations or directly in their own build trees.  A project may populate either the
       user  or  system  registry  (using its own means, see below) to refer to its location.  In
       either case the package should store at the registered location  a  Package  Configuration
       File    (<PackageName>Config.cmake)    and    optionally    a    Package    Version   File
       (<PackageName>ConfigVersion.cmake).

       The find_package() command searches the two package registries as two of the search  steps
       specified  in  its  documentation.  If it has sufficient permissions it also removes stale
       package registry entries that refer to directories that do not exist or do not  contain  a
       matching package configuration file.

   User Package Registry
       The  User  Package Registry is stored in a per-user location.  The export(PACKAGE) command
       may be used to register a  project  build  tree  in  the  user  package  registry.   CMake
       currently  provides  no  interface  to  add  install  trees  to the user package registry.
       Installers must be manually taught to register their packages if desired.

       On Windows the user package registry is stored in the Windows  registry  under  a  key  in
       HKEY_CURRENT_USER.

       A <PackageName> may appear under registry key:

          HKEY_CURRENT_USER\Software\Kitware\CMake\Packages\<PackageName>

       as  a  REG_SZ  value,  with  arbitrary  name,  that specifies the directory containing the
       package configuration file.

       On UNIX platforms the user package registry is stored in the  user  home  directory  under
       ~/.cmake/packages.  A <PackageName> may appear under the directory:

          ~/.cmake/packages/<PackageName>

       as  a  file,  with  arbitrary  name,  whose content specifies the directory containing the
       package configuration file.

   System Package Registry
       The System Package Registry is stored in a system-wide location.  CMake currently provides
       no interface to add to the system package registry.  Installers must be manually taught to
       register their packages if desired.

       On Windows the system package registry is stored in the Windows registry under  a  key  in
       HKEY_LOCAL_MACHINE.  A <PackageName> may appear under registry key:

          HKEY_LOCAL_MACHINE\Software\Kitware\CMake\Packages\<PackageName>

       as  a  REG_SZ  value,  with  arbitrary  name,  that specifies the directory containing the
       package configuration file.

       There is no system package registry on non-Windows platforms.

   Disabling the Package Registry
       In some cases using the Package Registries is not desirable. CMake allows one  to  disable
       them using the following variables:

          · CMAKE_EXPORT_NO_PACKAGE_REGISTRY disables the export(PACKAGE) command.

          · CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY  disables the User Package Registry in all the
            find_package() calls.

          · CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY disables the System Package Registry in
            all the find_package() calls.

   Package Registry Example
       A  simple  convention  for naming package registry entries is to use content hashes.  They
       are deterministic and unlikely to collide (export(PACKAGE) uses this approach).  The  name
       of  an  entry referencing a specific directory is simply the content hash of the directory
       path itself.

       If a project arranges for package registry entries to exist, such as:

          > reg query HKCU\Software\Kitware\CMake\Packages\MyPackage
          HKEY_CURRENT_USER\Software\Kitware\CMake\Packages\MyPackage
           45e7d55f13b87179bb12f907c8de6fc4 REG_SZ c:/Users/Me/Work/lib/cmake/MyPackage
           7b4a9844f681c80ce93190d4e3185db9 REG_SZ c:/Users/Me/Work/MyPackage-build

       or:

          $ cat ~/.cmake/packages/MyPackage/7d1fb77e07ce59a81bed093bbee945bd
          /home/me/work/lib/cmake/MyPackage
          $ cat ~/.cmake/packages/MyPackage/f92c1db873a1937f3100706657c63e07
          /home/me/work/MyPackage-build

       then the CMakeLists.txt code:

          find_package(MyPackage)

       will   search   the    registered    locations    for    package    configuration    files
       (MyPackageConfig.cmake).   The  search  order  among package registry entries for a single
       package is unspecified and the entry names (hashes  in  this  example)  have  no  meaning.
       Registered  locations  may contain package version files (MyPackageConfigVersion.cmake) to
       tell find_package() whether a specific location is suitable for the version requested.

   Package Registry Ownership
       Package registry entries are individually owned by the  project  installations  that  they
       reference.   A  package  installer  is  responsible  for  adding  its  own  entry  and the
       corresponding uninstaller is responsible for removing it.

       The export(PACKAGE) command populates the user package registry with  the  location  of  a
       project  build tree.  Build trees tend to be deleted by developers and have no “uninstall”
       event that could trigger removal of their entries.  In order to keep the registries  clean
       the  find_package()  command  automatically  removes stale entries it encounters if it has
       sufficient permissions.  CMake provides no interface to remove  an  entry  referencing  an
       existing  build  tree  once  export(PACKAGE)  has  been  invoked.  However, if the project
       removes its package configuration file from the build tree then the entry referencing  the
       location will be considered stale.

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