Thank you Bregma,
Your answer almost gave me a heart attack. For all the porting a need to do, I thought OpenGL would be a breeze.
Note though that all these libraries are only for setting up the GL context and getting a window to draw in, and some input (and possibly a few utility functions)
Those of course have to be OS specific.
All the actual rendering is the same standard OpenGL, as long as you use the same profile, so no need for heart attacks
(some drivers might have bugs, or allow some invalid behaviour that breaks on other drivers, but that is a separate story... )