if softmmu is not enabled, we disable by default fdt and
slirp as they are only used by -softmmu targets.

A side effect is the git submodules are not cloned
if they are not needed.

Clone and build can be forced with --enable-fdt and
--enable-slirp.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190621130544.18860-1-lvivier@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>