Add OSMesa (CDAT_BUILD_OFFSCREEN to enable).
Fix the library extensions for osmesa linking.
Fix variable name collision.
Disable OSMesa on macs.

See discussion on #888.
Disable BUILD_GUI when BUILD_OFFSCREEN is available.

Also simplify the logic for the offscreen option.
Merge pull request #888 from UV-CDAT/add_osmesa

Add OSMesa (CDAT_BUILD_OFFSCREEN to enable).

Works on Rhea but let's not forget it fails on Mac.