diff --git a/config.h.in b/config.h.in
index f8d8143c0..f3aadc1fd 100644
--- a/config.h.in
+++ b/config.h.in
@@ -90,6 +90,9 @@
 /* Define if you use AFS.  */
 #undef HAVE_AFS
 
+/* Define if you use flex instead of lex.  */
+#undef HAVE_FLEX
+
 /* Define if you have POSIX signals.  */
 #undef HAVE_SIGACTION
 #ifdef HAVE_SIGACTION