The Fast Light Tool Kit ("FLTK", pronounced "fulltick") is a
a cross-platform C++ GUI toolkit for UNIX(r)/Linux(r) (X11),
Microsoft(r) Windows(r), and MacOS(r) X. FLTK provides
modern GUI functionality without the bloat and supports 3D
graphics via OpenGL(r) and its built-in GLUT emulation. It
was originally developed by Mr. Bill Spitzak and is
currently maintained by a small group of developers across
the world with a central repository in the US.

For more information see README.txt:
https://github.com/fltk/fltk/blob/master/README.txt