FLTK (pronounced "fulltick") is a LGPL'd C++ graphical user interface
toolkit for X (UNIX), OpenGL, and Win32.  FLTK was designed to be small and
modular enough to be statically linked, but also works fine as a shared
library.  FLTK also includes FLUID, an interactive user interface builder
program.

This is version 1.1.  From the FLTK website:

  FLTK 1.1.x is the former stable development branch. It is no longer
  being maintained, and any bug fixes will be applied against FLTK
  1.3.x.

  We recommend using the active FLTK 1.3.0 branch instead.