build: use -Werror=implicit-function-declaration to detect build issues

There have been review requests that include a call to a non-existent
function. These problems should be detected as early as possible,
preferably before (automated) regression tests.

Bug: 990243
Change-Id: Idcf4d27f45a1a6177105d02a039363702eb8e3b9
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/5435
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>