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