Require a specific Autoconf version, instead of a lower bound only.

configure.in

 dnl Process this file with autoconf to produce a configure script.
-dnl $PostgreSQL: pgsql/configure.in,v 1.537 2007/11/16 04:29:45 scrappy Exp $
+dnl $PostgreSQL: pgsql/configure.in,v 1.538 2007/11/26 12:31:07 petere Exp $
 dnl
 dnl Developers, please strive to achieve this order:
 dnl
@@ -19,7 +19,7 @@ m4_pattern_forbid(^PGAC_)dnl to catch undefined macros
 
 AC_INIT([PostgreSQL], [8.3beta3], [pgsql-bugs@postgresql.org])
 
-AC_PREREQ(2.59)
+m4_if(m4_defn([m4_PACKAGE_VERSION]), [2.59], [], [m4_fatal([Autoconf version 2.59 is required])])
 AC_COPYRIGHT([Copyright (c) 1996-2007, PostgreSQL Global Development Group])
 AC_CONFIG_SRCDIR([src/backend/access/common/heaptuple.c])
 AC_CONFIG_AUX_DIR(config)