build: Better configure control & messages for xml output

Users were not given a correct indication to whether xml output was
going to be built on running configure as the only information was lost
in the checks output. This would lead to conditions where the user would
assume gluster was compiled with xml output, when it wasn't the case.

With this change, a new line is added to the end of the configure output
which indicates if xml output is being built or not. Also, a new
configure flag, '--disable-xml-output', has been introduced which can be
used to explicitly disable xml outpu from being built.

Change-Id: Ie80ba0372b4c3967a836769c61b60263c8e5e350
BUG: 947226
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/4793
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>