Updating to Autoconf 2.5x

[DJ Delorie]

> I guess the royal ``we'' here is GCC?

A number of the configury maintainers are shared across projects, so
more than just gcc was represented there.  But there are a *lot* of
projects that will be affected by this.

> How does nathaniel and dj's work fit into this - its kind of a logical 
> next step.

Nathaniel is working on moving bootstrapping to the toplevel stuff, so
he's off on a branch.  I'm all for the conversion.