Start using @rpath for MacOSX correctly

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

Change-Id: If15a7dd1b0421b2c2f1a870edfca5fc467911ca1
Reviewed-on: http://review.couchbase.org/43397
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>