Check for the sys/cdefs.h header which on some systems defines the __warn_references macro.
diff --git a/config.h.in b/config.h.in
index 2dad0d9..34ad9a8 100644
--- a/config.h.in
+++ b/config.h.in
@@ -65,6 +65,9 @@
 /* Define to 1 if you have the <syslog.h> header file. */
 #undef HAVE_SYSLOG_H
 
+/* Define to 1 if you have the <sys/cdefs.h> header file. */
+#undef HAVE_SYS_CDEFS_H
+
 /* Define to 1 if you have the <sys/param.h> header file. */
 #undef HAVE_SYS_PARAM_H