OS2/ directory contains os2 specific Imakefile and shared
     library stuff.

If your system has OpenGL, you must rename GL/ directory
to something else in order to use the real OpenGL. GL/
directory here is mesa headers.

OpenGL canvas gl.c by default is compiled into the static library using
OpenGL on SGI and Mesa on other platforms. Shared library might not
contain gl.o on some platforms. You can always add it by compiling
gl.c here or compiling gl.c here and link it with your application. 

If you use the gl.o for Mesa from the pre-built static library, you need
Mesa 2.0

../DEMOS contains a few demo programs that use OpenGL/Mesa