fluid

fluid [ -c [ -o code-filename -h header-filename ] ] [ filename.fl ]

fluid is an interactive GUI designer for FLTK. When run with no arguments or with a filename, fluid will display the GUI hierarchy and any windows defined in the file. Functions, classes, windows, and GUI components can be manipulated as needed.

When used with the -c option, fluid will create the necessary C++ header and code files in the current directory. You can override the default extensions, filenames, and directories using the -o and -h options.

fltk-config(1), fltk(3)
FLTK Programming Manual, Chapter 9
FLTK Web Site, http://www.fltk.org/

Bill Spitzak and others.