Fix the networking stuff on (current) OS X

OS X's headers split up the current and old (BIND 4) nameserver stuff
slightly differently than Linux does, but explicitly including
arpa/nameser_compat.h does the right thing on both. Part of #580301

gio/ginetaddress.c

@@ -22,6 +22,9 @@
  */
 
 #include <config.h>
+
+#include <string.h>
+
 #include <glib.h>
 
 #include "ginetaddress.h"

gio/gnetworkingprivate.h

@@ -31,10 +31,12 @@
 
 #else /* !G_OS_WIN32 */
 
-#define BIND_4_COMPAT
-
 #include <arpa/inet.h>
 #include <arpa/nameser.h>
+#if HAVE_ARPA_NAMESER_COMPAT_H
+#include <arpa/nameser_compat.h>
+#endif
+
 /* We're supposed to define _GNU_SOURCE to get EAI_NODATA, but that
  * won't actually work since <features.h> has already been included at
  * this point. So we define __USE_GNU instead.