-- PLEASE NOTICE ---> this version is meant to coexist with autoconf-2.5x;
to that end, the various binaries are actually linked to a script which
decides which version to execute. It tries to be clever and will execute
2.5x if `configure.ac' is present, or if `configure.in' contains AC_PREREQ
and the value's 3 first letters are stringwise greater than '2.1'. If
anything fails, you can manually select 2.13 by providing the environment
variable WANT_AUTOCONF_2_1 set to `1' (use WANT_AUTOCONF_2_5 if you want
the 2.5x version).