mapi/glapi: Use ElementTree instead of libxml2.

mapi/glapi: Use ElementTree instead of libxml2.

It is quite hard to meet the dependency of the libxml2 python bindings
outside Linux, and in particularly on MacOSX; whereas ElementTree is
part of Python's standard library.  ElementTree is more limited than
libxml2: no DTD verification, defaults from DTD, or XInclude support,
but none of these limitations is serious enough to justify using
libxml2.

In fact, it was easier to refactor the code to use ElementTree than to
try to get libxml2 python bindings.

In the process, gl_item_factory class was refactored so that there is
one method for each kind of object to be created, as it simplifies
things substantially.

I confirmed that precisely the same output is generated for GL/GLX/GLES.

v2: Remove m4/ax_python_module.m4 as suggested by Matt Turner.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>