at_top_builddir -> at_top_build_prefix / Autotest Versioning

[Noah Misch]

On 2005-05-20, at_top_builddir, a variable `configure' passes to `testsuite' via
`atconfig', changed names to at_top_build_prefix.  Consequently, `configure'
generated by `autoconf' predating the change do not properly configure
`testsuite' generated by `autom4te' postdating the change, and vice-versa.

With the manifest breakage, `testsuite' treats its suite as freestanding and so
does not augment PATH.  The suite therefore tests already-installed copies of
the subject programs instead of the copies the user just built.

This especially touches Autoconf, since we always build `testsuite' with the new
`autom4te', but Automake's `configure' update rule uses `autoconf' from one's
PATH.  It will affect developers of other packages using Autotest when they
rebuild `configure' with a different `autoconf' without also rebuilding their
test suite.

We could avoid subtle breakage by encoding a version number in `atconfig' and
`testsuite' and comparing them.  Is there an easier/better way?

In the mean time, be sure to install a post-2005-05-20 Autoconf before
rebuilding Autoconf's `configure'.