This commit ensures the file defining VTK_USE_{CARBON|COCOA} is included
and fixes the following build error caught by Slicer dashboard:

// ---------
In file included from /Users/kitware/Dashboards/Experimental/Slicer-0/Modules/Loadable/VolumeRendering/VolumeRenderingReplacements/vtkSlicerOpenGLRayCastImageDisplayHelper.cxx:25:
/Users/kitware/Dashboards/Experimental/Slicer-0-build/VTKv6/Rendering/OpenGL2/vtkOpenGL.h:26:11: fatal error: 'GL/gl.h' file not found

          ^
1 error generated.
// ---------

Change-Id: I34b62ab1ab6954ebba6ca91b6c99dd00b30f1957