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.
Auto detect git protocol

No longer we will need to run cmake twice
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.
Merge pull request #914 from UV-CDAT/auto_detect_git_protocol

Auto detect git protocol
Merge branch 'master' into optimize_vtk