* configure.ac:
* clutter/cogl/gl/cogl-defines.h.in: The OS X backend has a funny
OpenGL header location, so it basically did break every application
including clutter/cogl.h because the GL header inclusion depended
on symbols defined into Clutter's config.h. Now, we define the
GL header into the configure template and use it to create the
cogl-defines.h file cogl.h includes.
