Thank you for your quick responses.
I am aware of the implications of machine specific code etc etc.
I have already written software which has almost no differences in the C source code between the linux and windows versions, I then compile it using GCC on linux, and GCC MinGW for windows. I have yet to write any programs for Mac, I dont even own one (or installed MacOS on a PC).
Back to the original notion, could you elaborate on the OpenGL concept for creating a skinned GUI.