Move installing osmesa.pc to drivers/osmesa, where it belongs better

This also restores the installation of gl.pc if we are building osmesa at the
same time as libGL, which was broken in commit 39785488

 when the .pc
installation was converted to automake

v2:
Remove HAVE_OSMESA_DRIVER automake conditional, it's now pointless as we
will only be building in the drivers/osmesa directory if the condition it
checked was true.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>