[CMake][Release] Use @rpath on OS X if possible

CMake >= 2.8.12 can optionally use @rpath to embed dynamic linker paths
into shared libraries on OS X. This greatly improves reliability with
libraries in non-standard locations and should definitely be turned on
by default.

(cherry picked from commit 13fc4020)
Signed-off-by: Steffen Müthing <muething@dune-project.org>