api: glusterfs-api.pc.in use rpath (-Wl,-R...) (backport BZ 1002220)

Change-Id: Ie36d7c3c058c2e738b0fb1ffae116ed43d573474
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
BUG: 1004751
Reviewed-on: http://review.gluster.org/5822
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>