ENH: Add VTK_QT_VERSION to the VTK global config. options.

By doing this we can determine which version of Qt was used
  when VTK was built. Thus in CMakeFiles.txt we can do something
  like this:

    if(VTK_QT_VERSION VERSION_GREATER "4")
      # Do something specific for Qt5
    else()
      # Do something specific for Qt4
    endif()

This allows Qt4 specific or Qt5 specific builds when using Qt
  with VTK.

Change-Id: Ia0e1119c5e824a84478e1137b31d19bb868afda0
Signed-off-by: Andrew Maclean <andrew.amaclean@gmail.com>
Merge topic 'make_vtk_qt_version_global' into master

c055be00 ENH: Add VTK_QT_VERSION to the VTK global config. options.