Start using @rpath for MacOSX correctly

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

Change-Id: I2ddd1c79ce81a7f19d4b49d87fb855b52f1af545
Reviewed-on: http://review.couchbase.org/43396
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>