When building OSMesa and xlib GL, the resulting OSMesa would be linked
against libGL instead of the internal mesa libraries. However, when
building with -fvisibility=hidden, some of the internal functions used
in OSMesa could not be resolved through libGL.

Instead, always build OSMesa standalone without linking against libGL.
This has the advantage that OSMesa is always built the same way, but it
means that disk space is wasted when libGL is installed since both
libraries will contain the internal objects.

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>
Tested-by: Tom Fogal <tfogal@alumni.unh.edu>