diff --git a/acinclude.m4 b/acinclude.m4
index cd02300530..4ac611dd23 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -155,7 +155,7 @@ AC_DEFUN([AC_TRY_LDFLAGS],
 #   and this notice are preserved.  This file is offered as-is, without any
 #   warranty.
 
-#serial 3
+serial 3
 
 AC_DEFUN([AX_GCC_BUILTIN], [
     AS_VAR_PUSHDEF([ac_var], [ax_cv_have_$1])