Remove duplicates from the include dirs

Having duplicates dramatically increase the command line arguments
that can become close to the maximum limit with ITK and VTK.
Merge branch 'remove-include-duplicates'

* remove-include-duplicates:
  Remove duplicates from the include dirs
Do not link with VTK targets that don't exist
Merge branch 'fix-vtk6-linking'

* fix-vtk6-linking:
  Do not link with VTK targets that don't exist
Don't use set_directory_properties in UseCTK.cmake

It does not do the same as include_directories. Instead, do not add
libraries that already exist.
Merge branch 'remove-include-duplicates'

* remove-include-duplicates:
  Don't use set_directory_properties in UseCTK.cmake