[RFC] Add dummy toplevel configure.ac

[Zack Weinberg]

> > Does autoconf have something like #error that you could use here to
> > actually just issue that as an error message?
>
> Unfortunately I didn't find anything here...  m4 might have.

You want AC_FATAL ...

AC_FATAL([error: toplevel configure should be created using autoconf-2.13])

I don't know when it was added, but it works with 2.59, and has the nice
property of not clobbering the existing 'configure'.

zw