The GLEW headers are enough, and what we actually use in these
places. In addition to handling GL extension things in its dynamic
fashion, GLEW headers also have declarations for standard,
non-extension, OpenGL API, including xgl and wgl ones.

Most likely we don't need mesa_headers on Windows or OS X either, and
can drop them completely.

Change-Id: Ic0d8d6238c862f8fe4a74e99e95344dcbf540980