glx: permit building with older protocol headers

glx: permit building with older protocol headers

I'd like to be able to build mesa on current distro releases without
having to upgrade from the standard dri2proto and glproto headers.  With
this change I'm able to build on ancient releases such as Ubuntu 9-10...

In general, it would be nice to be able to build-test mesa to check for
unintended breakages without having to follow the external dependencies
of every group working on the codebase.

Seems to introduce no changes to the build of libglapi.a when tested against
new versions of the headers.