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 a version 1.4 development snapshot.  From the FLTK website:

  FLTK-1.4.x [under active development]
  Work on FLTK-1.4.x began at the start of 2016 because it was necessary to
  break the FLTK-1.3.x ABI in order to fix some long outstanding problems
  and to add new features. Resetting the ABI also allowed the removal of
  200+ areas of conditional compilation involving ABI guards, greatly
  simplifying the code.
  FLTK-1.4.x already boasts a completely new abstract device driver class
  hierarchy to isolate platform specific code in one place instead of using
  #ifdef everywhere, and to make it easier to port to new platforms.
