Allow using VMTK functions from a C++ Slicer module

[cpinter]

Currently it does not seem possible to use VMTK functions from a C++ loadable Slicer module. It would be needed to expose the *_INCLUDE_DIRS and *_LIBRARY_DIRS CMake variables in SlicerVMTKConfig.cmake.

For an example see how the SlicerCollaboration extension uses the SlicerOpenIGTLink extension (that is also a superbuild extension just like SlicerVMTK).

[lassoan]

Good idea. If you need any of these then someone could add these and send a pull request.