MinGW uses autoconf 2.56


git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@84 632fc199-4ca6-4c93-a231-07263d6284db
diff --git a/configure.ac b/configure.ac
index cd32445..2b6a87e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,7 +1,7 @@
 #                                               -*- Autoconf -*-
 # Process this file with autoconf to produce a configure script.
 
-AC_PREREQ([2.57])
+AC_PREREQ([2.56])
 AC_INIT([libjpeg], [6.b])
 
 AM_INIT_AUTOMAKE([-Wall foreign dist-bzip2])