Fails to build on NetBSD

  CC      /home/qemu/qemu-test.aOSubt/build/slirp/src/slirp.o
In file included from /usr/pkg/lib/glib-2.0/include/glibconfig.h:9:0,
                 from /usr/pkg/include/glib-2.0/glib/gtypes.h:32,
                 from /usr/pkg/include/glib-2.0/glib/galloca.h:32,
                 from /usr/pkg/include/glib-2.0/glib.h:30,
                 from /home/qemu/qemu-test.aOSubt/src/slirp/src/util.h:27,
                 from /home/qemu/qemu-test.aOSubt/src/slirp/src/slirp.h:47,
                 from /home/qemu/qemu-test.aOSubt/src/slirp/src/slirp.c:25:
/home/qemu/qemu-test.aOSubt/src/slirp/src/slirp.c: In function 'slirp_new':
/home/qemu/qemu-test.aOSubt/src/slirp/src/slirp.c:277:29: error: 'SlirpConfig {aka const struct SlirpConfig}' has no member named 'if_data'
     g_return_val_if_fail(cfg->if_mtu >= IF_MTU_MIN || cfg->if_mtu == 0, NULL);
                             ^

This is due to some archaic #define if_mtu in #include <net/if.h>