cbdeps: Prepend @rpath to jemalloc install name

Add @rpath to the libjemalloc.2.dylib install name on OS X. This
allows programs which link against jemalloc to search in their RPATH
for it.

Corresponds to cbdeps build: 5d9732f-cb2

Change-Id: Ia78aa8ac435fa16a16f6d8a8029db19054cbf2fa
Reviewed-on: http://review.couchbase.org/43193
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Dave Rigby <daver@couchbase.com>