Requiring Autoconf 2.61

[Pavel Roskin]

Hello!

I suggest that we require Autoconf 2.61 in configure.ac.  It has been
out for over a year.  It's easy to install locally without breaking
anything.  It generates more portable configure scripts than older
version of Autoconf (at least it's supposed to, unless there are
regressions).

I see that configure scripts generated by Autoconf 2.59 and 2.61 are
committed to the GRUB CVS alternately.  If the current configure is made
by Autoconf 2.59, and it's regenerated on my system by Autoconf 2.61, I
see unnecessary differences.

If somebody has an older version of Autoconf and doesn't want to
upgrade, there is always an option to run "touch configure" to provent
make from trying to regenerate it.

In addition to configure, there are also subtle differences in
config.h.in, which is generated by autoheader.

-- 
Regards,
Pavel Roskin