Add VTK_USE_COCOA and VTK_USE_X to VTK config file
When building LookingGlassVTKModule externally (thanks to PR Kitware/LookingGlassVTKModule#9, it fails to include
vtkCocoaLookingGlassRenderWindow.h (see here and issue KitwareMedical/SlicerLookingGlass#2)
Moving forward, I would like to expose option like
VTK_USE_X into a config file.
What is the CMake equivalent of