mesa: update glext.h to version 20130708

mesa: update glext.h to version 20130708

This update fixes the problem with duplicated typedefs for
GLclampf and GLclampd in the previous version.

It also changes some parameter types for glDebugMessageCallbackARB()
and glTransformFeedbackVaryingsEXT().

Note we should someday update the glapi-gen code so that it
understands void pointer parameters.  Currently, the Python code
only understands "GLvoid *" but not "void *".  Luckily, the
compilers don't seem to complain about mixing GLvoid and void.