Some GLSL-related function prototypes are GLES2-only: GL implementations
are not required to provide them.
While Mesa is perfectly happy to return a dummy function pointer for
functions it doesn't support, other platforms are more picky, and will
return NULL.
In this particular case, this commit fixes GLSL support on OSX.
https://bugzilla.gnome.org/show_bug.cgi?id=668856
Reviewed-by: Neil Roberts <neil@linux.intel.com>
417878538b