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
1d needed to return empty dict

merge seems to have put back old genGrid line for vector case (or I might have never commented it out)
Merge branch 'master' into optimize_vtk
Merge pull request #893 from UV-CDAT/optimize_vtk

Optimize vtk
Merge remote-tracking branch 'origin/master' into vtk_labeled_contour_for_aashish

Conflicts:
	Packages/vcs/Lib/VTKPlots.py