[vtkusers] Installing vtkExportKit.cmake and vtkKit.cmake.in

[orion at cora.nwra.com]

Xdmf make use of vtkExportKit.cmake and vtkKit.cmake.in as part of its
install process.  So it seems like vtk should install these into
/usr/lib/vtk-VER/CMake with the other files there.  The one hiccup seems
to be the use of %{VTK_INSTALL_PACKAGE_DIR_CM24} in vtkExportKit.cmake -
this variable would be need to resolved to the proper value at install
time.

Thoughts?

- Orion