Start using @rpath for MacOSX correctly

Introduced in CMake 3 there is a new policy for dealing with
rpath.

Change-Id: I511da057eb794983617dec983a6a49114082bd28
Reviewed-on: http://review.couchbase.org/43398
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>