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.