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.