The Fast Light Tool Kit is a cross-platform C++ GUI toolkit for
UNIX®/Linux® (X11 or Wayland), Microsoft® Windows®, and macOS®.
FLTK provides modern GUI functionality without bloat and
supports 3D graphics via OpenGL® 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 on GitHub.

    https://www.fltk.org/
    https://github.com/fltk/fltk/

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