diff --git a/src/config.in b/src/config.in
index abf084ce735..693dea8dc6c 100644
--- a/src/config.in
+++ b/src/config.in
@@ -211,6 +211,8 @@ Boston, MA 02111-1307, USA.  */
 
 #undef HAVE_ALLOCA_H
 
+#undef HAVE_DEV_PTMX
+
 #undef HAVE_GETTIMEOFDAY
 /* If we don't have gettimeofday,
    the test for GETTIMEOFDAY_ONE_ARGUMENT may succeed,