the implicit sys/sysmacros.h include via sys/types.h is being deprecated.  easy fix is to include sys/sysmacros.h directly which should work under Linux C libs.  if portability is a concern, then AC_HEADER_MAJOR can be used.

https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Particular-Headers.html

--- a/metadata/meta-daemon.c
+++ b/metadata/meta-daemon.c
@@ -27,6 +27,7 @@
 #include <glib/gstdio.h>
 #include <locale.h>
 #include <stdlib.h>
+#include <sys/sysmacros.h>
 #include "metatree.h"
 #include "gvfsdaemonprotocol.h"
 #include "metadata-dbus.h"
--- a/metadata/metatree.c
+++ b/metadata/metatree.c
@@ -1,4 +1,5 @@
 #include "config.h"
+#include <sys/sysmacros.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/mman.h>