This allows source files to use the VTK-m version.

Note that Configure.h does not include the version. This is intentional.
First, I don't want the entirety of VTK-m to recompile every time the
version is updated (which is every time a commit happens in git).
Second, it is important that external project source files be able to
get the VTK-m version without actually loading any VTK-m code.