Enable CMake policy CMP0042, if supported:

> CMake 2.8.12 and newer has support for using ``@rpath`` in a target's
> install name.  This was enabled by setting the target property
> ``MACOSX_RPATH``.  The ``@rpath`` in an install name is a more
> flexible and powerful mechanism than ``@executable_path`` or
> ``@loader_path`` for locating shared libraries.