Adjust configure.ac to honor existing CFLAGS env variable

Change-Id: I43a8defc7a8c0c2d391e7bd2c0b7bebfcc522a2a
BUG: 961892
Signed-off-by: Justin Clift <jclift@redhat.com>
Reviewed-on: http://review.gluster.org/4979
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>