autotools: use localstatedir, not runstatedir

.. runstatedir is only present in autoconf 2.70+

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/Makefile.am b/Makefile.am
index 8e0d140..e494d27 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -2,7 +2,7 @@
 
 nbd_proxy_CPPFLAGS = \
 	$(JSON_CFLAGS) \
-	-DRUNSTATEDIR=\"$(runstatedir)\" \
+	-DRUNSTATEDIR=\"$(localstatedir)/run\" \
 	-DSYSCONFDIR=\"$(sysconfdir)\"
 
 nbd_proxy_LDADD = \