configure: check for -Wendif-labels support

Older gcc compilers do not support -Wendif-labels, so move it from the
hardcoded list to the dynamically detected list.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
