Small refactor. Move several utility functions from glX_XML.py to

Small refactor.  Move several utility functions from glX_XML.py to
FilterGLAPISpecBase (in gl_XML.py).  When these functions are used to emit
common #define lines, the will automatically be undefined in
FilterGLAPISpecBase::printFooter if header_tag is set (i.e., the file is a
header file).

These changes do not modify the generated code.