Add a check to ensure the correct autoconf version is used

for generating the configure script.

Original idea by Martin von Löwis.

configure.in

@@ -6,8 +6,16 @@ dnl NOTE: autoconf 2.64 doesn't seem to work (use 2.61).
 # Set VERSION so we only need to edit in one place (i.e., here)
 m4_define(PYTHON_VERSION, 2.7)
 
+dnl Some m4 magic to ensure that the configure script is generated
+dnl by the correct autoconf version.
+m4_define([version_required],
+[m4_if(m4_version_compare(m4_defn([m4_PACKAGE_VERSION]), [$1]), 0,
+       [],
+       [m4_fatal([Autoconf version $1 is required for Python], 63)])
+])
+version_required(2.61)
+
 AC_REVISION($Revision$)
-AC_PREREQ(2.61)
 AC_INIT(python, PYTHON_VERSION, http://bugs.python.org/)
 AC_CONFIG_SRCDIR([Include/object.h])
 AC_CONFIG_HEADER(pyconfig.h)