glapi: Generate GL API marshalling code from the XML.

glapi: Generate GL API marshalling code from the XML.

This is not yet used in the build, just generated.

v2: Add missing build dependencies.
v3: Avoid mixing declarations and code, remove logic for avoiding emitting
    code that the compiler's optimizer can deal with anyway.
v4: (Timothy Arceri) move safe_mul() genereation here from a later patch.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Acked-by: Marek Olšák <maraeo@gmail.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Tested-by: Mike Lothian <mike@fireburn.co.uk>