[CMake] if/else statment based on VTK version

[Kathryn Hayes]

Hi,

We're building teem against VTK 4 and VTK 5, and it looks like we're going
to build it with different flags depending on the VTK version to get it to
compile.  Is there a good way to test for the VTK version in CMake so we
can use the same CMakeLists.txt for both cases?

Thanks,

Katie