Refactor the code to emit multiple-inclusion protection to

Refactor the code to emit multiple-inclusion protection to
FilterGLAPISpecBase.  Since the size_h mode of glX_proto_size.py will be
used to generate multiple header files, add an option to specify the define
that is used for multiple-inclusion protection.

The changes to the header files in this commit are just a side-effect of the
changes to the Python scripts.