[vtkusers] building without libGL

[Kyle Cronan]

Hi,

I'm currently using VTK 5.4 on a web server with OSMesa.  I was able
to build 5.4 without any libGL installed on the system at all, and in
fact, if I recall correctly, I had to make sure it was removed and
reconfigure with CMake before I was able to get OSMesa working
properly (OSMesa didn't like being linked in when regular GL was also
present).

Now I would like to upgrade to the 5.6 series to take advantage of the
new charts.  But when I try to configure VTK 5.6.1 I get this error:

 CMake Error: The following variables are used in this project, but
they are set to NOTFOUND.
 Please set them or make sure they are set and tested correctly in the
CMake files:
 OPENGL_gl_LIBRARY (ADVANCED)
     linked by target "vtkVolumeRendering" in directory
/home/kyle/VTK/VolumeRendering
     linked by target "vtkWidgets" in directory /home/kyle/VTK/Widgets

The only way to get a build going seems to be to turn off rendering
entirely.  Recommendations?  Is this a known regression?

Note that as far as I know I don't actually use either of those kits.

Thanks,
Kyle Cronan