ENH: provide config setup for mesa with llvm (issue #1164)

- If using a non-clang compiler suite (gcc, intel, etc) the additional
  lbraries required for mesa with llvm pipelines may not be found.

  Provide a mesa_llvm configuration with in the 'vtk' config file.
  Can use the usual types of settings

     * mesa_llvm=llvm-4.0.1
     * mesa_llvm=none
     * mesa_llvm=system