glapi: Generate xserver glapi sources in the mesa tree

glapi: Generate xserver glapi sources in the mesa tree

Instead of generating the glapi sources for the xserver and commiting
them to the xserver tree, we can keep them in the mesa tree and change
the xserver build to use the files from the mesa tree.

This makes the xserver glx build more robust as it reduces the chances
for mismatches of the glX API used in the xserver vs. in mesa.