Provide a more helpful error message when there is an autoconf version

mismatch;  backpatch.

configure.in

@@ -1,5 +1,5 @@
 dnl Process this file with autoconf to produce a configure script.
-dnl $PostgreSQL: pgsql/configure.in,v 1.431.2.20 2007/11/26 12:27:03 petere Exp $
+dnl $PostgreSQL: pgsql/configure.in,v 1.431.2.21 2007/12/31 17:28:12 momjian Exp $
 dnl
 dnl Developers, please strive to achieve this order:
 dnl
@@ -19,7 +19,10 @@ m4_pattern_forbid(^PGAC_)dnl to catch undefined macros
 
 AC_INIT([PostgreSQL], [8.1.10], [pgsql-bugs@postgresql.org])
 
-m4_if(m4_defn([m4_PACKAGE_VERSION]), [2.59], [], [m4_fatal([Autoconf version 2.59 is required])])
+m4_if(m4_defn([m4_PACKAGE_VERSION]), [2.59], [], [m4_fatal([Autoconf version 2.59 is required.
+Untested combinations of 'autoconf' and PostgreSQL versions are not
+recommended.  You can remove the check from 'configure.in' but it is then
+your responsibility whether the result works or not.])])
 AC_COPYRIGHT([Copyright (c) 1996-2005, PostgreSQL Global Development Group])
 AC_CONFIG_SRCDIR([src/backend/access/common/heaptuple.c])
 AC_CONFIG_AUX_DIR(config)