build: properly handle --without-{xml,json}-parsing

This patch fixes how --without-{xml,json}-parsing (and subsequently
--with-{xml,json}-parsing=no) is handled.  Prior to this,
--without-{xml,json}-parsing actually resulted in libnftables being
built with that parsing enabled.

Signed-off-by: Douglas Freed <dwfreed@mtu.edu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>