build: handle dependencies for syslog properly

This patch conditionally enables rsyslog dependencies based on syslog
build option.

Change-Id: I2168f29a03504ec839c3048ce112e8dfb2226aa0
BUG: 928648
Signed-off-by: Bala.FA <barumuga@redhat.com>
Reviewed-on: http://review.gluster.org/5684
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>