libglusterfs: Do not redefine AT_SYMLINK_NOFOLLOW on Darwin

Change-Id: I6c31b0a01da4b2ad05d4df67418e917c2774faa9
BUG: 1089172
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/8943
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>