--- a/m4/unlocked-io.m4
+++ b/m4/unlocked-io.m4
@@ -1,4 +1,21 @@
-#serial 9
+# unlocked-io.m4 serial 10
+
+# Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004 Free Software
+# Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2, or (at your option)
+# any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 dnl From Jim Meyering.
 dnl
@@ -10,6 +27,11 @@
 
 AC_DEFUN([gl_FUNC_GLIBC_UNLOCKED_IO],
 [
+  AC_DEFINE([USE_UNLOCKED_IO], 1,
+    [Define to 1 if you want getc etc. to use unlocked I/O if available.
+     Unlocked I/O can improve performance in unithreaded apps,
+     but it is not safe for multithreaded apps.])
+
   dnl Persuade glibc and Solaris <stdio.h> to declare
   dnl fgets_unlocked(), fputs_unlocked() etc.
   AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])