We teach VTK to install a package version file 'VTKConfigVersion.cmake'
next to the package config file 'VTKConfig.cmake'.  This allows code
like

  find_package(VTK 5.5)

in user projects to find only compatible VTK versions.  Enforcement of
the version check will only work when the user project runs with the
FindVTK.cmake module from CMake 2.8 or higher.  The package version file
will simply be ignored by older CMake versions.