build: Avoid gcc pragmas when clang is enabled

Change-Id: I86544a9bfd24ef1a69315c0f6c3abcdd9aeb2c69
BUG: 1089172
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/7932
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>