IUP
 
Portable toolkit for building graphical user interfaces.

Website: http://www.tecgraf.puc-rio.br/iup/
Platforms supported: Win32, Linux
Headers to include: IUP/iup.bi, IUP3/iup.bi
Header version: 2.3.0, 3.5 (Lua part missing at the moment)
Example Usage: yes, in examples/libraries/IUP/