Add (default disabled) --enable-werror option to configure

Useful for example during development or automated builds, to make the
build error out if new warnings are introduced.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1401359947-31144-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8747
Signed-off-by: Gert Doering <gert@greenie.muc.de>