GCC 4.6 compatibility: no `override` keyword.
Fixed OpenGL ES 2.0 compatibility.

Issues related to lack of glDrawRangeElements().
Include OpenGL headers instead of trying to be smart.

I hoped the typedef was the same everywhere, but apparently ES has
something else, which is typedef'd recursively and I don't want to
maintain the consequences when porting elsewhere.
Disable building of Audio library in Native Client for now.

We now support NaCl 25, which doesn't have bundled OpenAL and I don't
feel like bumping the version to 27 yet.