updates for Mesa 7.8.2:
- build glsl as a host tool
- add the glsl objects to libmesa
- add/remove new/deleted sources for various components
- adjust the libmesa/libGL builds to pull in the new glsl objects
- re-apply the BUILDSYMLINK hack for eval.c/pixel.c/pixelstore.c
- use glsl to build the slang headers on the fly