The usual...
diff --git a/config.h.in b/config.h.in
index 903db9c..7c17b83 100644
--- a/config.h.in
+++ b/config.h.in
@@ -86,6 +86,9 @@
 /* Define this if your time.h defines altzone */
 #undef HAVE_ALTZONE
 
+/* Define this if you have a K&R style C preprocessor */
+#undef HAVE_OLD_CPP
+
 /* Define if your compiler supports function prototypes */
 #undef HAVE_PROTOTYPES