autotools: Add macros for AX_APPEND_COMPILE_FLAGS

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/configure.ac b/configure.ac
index 2e48ec4..9bd1841 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,5 +1,6 @@
 AC_PREREQ([2.69])
 AC_INIT([nbd-proxy], 1.0, [jk@ozlabs.org])
+AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_HEADERS([config.h])
 AM_INIT_AUTOMAKE([subdir-objects -Wall -Werror foreign dist-xz])
 AM_SILENT_RULES([yes])