|FLTK is a cross platform user interface library, originally written by Bill Spitzak to improve on and replace the Forms Library that was often used for Unix and SGI apps in the early 90’s.
Bill wrote FLTK so he could improve his compositing software NUKE. When I joined the Nuke team in 1995, I was in the process of rewriting Movido. FLTK was the perfect choice, so I ported FLTK to MSWindows, then classic Mac OS, and later to OS X (now macOS).
If Qt is too big of a library for you to write cross platform code, check out FLTK here.