Set CMP0042 explicitly with CMake 3.0 and above

Some host projects no longer set `CMAKE_MACOSX_RPATH` explicitly due to
requiring CMake 3.0 or above, and now see CMP0042 warnings.  Set the
policy explicitly in KWSys to avoid the warnings.  Host projects can
still set `CMAKE_MACOSX_RPATH` to `0` to get the old behavior.

Suggested-by: Ken Martin <ken.martin@kitware.com>