GCC 3.x requires autoconf 2.13

[DJ Delorie]

> Although the best way to fix this problem is to get things working with the
> latest autoconf -- if that's low priority or is going to take a long time, I
> can see where adding a comment at the top of configure.in, aclocal.m4, or
> even adding a rule that checks the autoconf version and breaks in a
> meaningful way would be a big help to new developers who aren't in the know.

We've been talking about how to work on solving this, so I think it
would be premature to add such workarounds at the moment.  If it
stalls, though, that might be a good idea.