Extend OpenGL API list to all functions in Khronos gl.xml

Here we add the gl.xml file downloaded from:

https://cvs.khronos.org/svn/repos/ogl/trunk/doc/registry/public/api/gl.xml

Along with a script and Makefile to convert it to the list form
expected by glaze.c.

Note: The Makefile to perform the conversion is not called by
default. Instead, the result of the conversion (gl.def) is committed
directly to the source repository. This makes it convenient, (the
general user does not need to install xmlstarlet, for example), while
a developer that does need to update gl.xml can simply invoke "make
clean; make" in the specs directory to rebuild gl.def.