COMP: Fix VTK6 link error on MacOSX 10.6. Fixes #3741

To reduce the number of libraries passed to the linker, this commit build
VTK6 with the VTK_ENABLE_KITS option enabled.

If building on MacOSX 10.6 without CMake 3.0, it also updated the build
system to output a fatal error message explaining that a more recent
version of CMake is required.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23509 3bd1e089-480b-0410-8dfb-8563597acbee