Bug 913 - cogl fails to build on OSX
* clutter/cogl/gl/cogl-fbo.c: define GL_READ_FRAMEBUFFER_EXT and
GL_DRAW_FRAMEBUFFER_EXT if not defined in the build system. The
#ifdef can fail if they're not #defines but variables or enums.
As the values are supposed to be industry standard even then
it shouldn't have ill effects.
3d034ff6aa