configure.in -> configure.ac in make-stds.texi

[Paul Eggert]

As Autoconf will start deprecating the file name 'configure.in' soon,
make-stds.texi should change to talk about the current name 'configure.ac'.
Here's the obvious patch.

--- make-stds.texi      23 Nov 2010 00:24:04 -0000      1.63
+++ make-stds.texi      9 Jul 2012 08:21:35 -0000
@@ -602,7 +602,7 @@
 
 If you are using Autoconf, write the default as @samp{@@lispdir@@}.
 In order to make @samp{@@lispdir@@} work, you need the following lines
-in your @file{configure.in} file:
+in your @file{configure.ac} file:
                                                                                
 @example
 lispdir='address@hidden@}/emacs/site-lisp'