Use the new (-no)-wmf-backend configuration option

Don't build the wmf backend when configure was executed
with the -no-wmf-backend option.

Change-Id: Ia047137c15c7aac302092e9f77b961c6e584bb86
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>