BUG: OSMESA configuration flags can cause link errors.
This fixes:
VTK_OPENGL_HAS_OSMESA on
VTK_USE_X on
caused link errors in Rendering/OpenGL as some of the cxx
files contained code ifdef using VTK_OPENGL_HAS_OSMESA instead of
VTK_USE_OSMESA
In the past, one could link against both MESA and OSMESA and use OSMESA only
for off-screen rendering. In recent versions of MESA this is not the case anymore.