Log Message:
Hopefully finally get this right:
Default Mesa Version should be 3.4.2, so that people who don't care and have
GL in XF don't need to build the whole thing.  If MESA_REQD is set to 5.0.1
(as, for example, by MesaDemos), require version 5.0.1 of MesaLib, gl* etc.

If libGL.so.5 is found to be present on the system, force MESA_REQD to be
5.0.1 to avoid accidental downgrades if MESA_REQD was not specified before-
hand.

Changes reviewed by tron@ (thanks!).