automake, aclocal: honour configure-time AUTOCONF and AUTOM4TE

automake, aclocal: honour configure-time AUTOCONF and AUTOM4TE

Currently, the Automake's own configure script allow definition
of AUTOCONF and AUTOM4TE, expected to point respectively to an
autoconf and autom4te programs.  But while these definitions are
honoured in the Automake's build systems and test suite, they
were *not* honoured in the generated `automake' and `aclocal'
scripts.  This behaviour, apart from being wrong in that it does
not allow the user enough freedom in choosing his tools, also
caused inconsistencies in the test suite, brining to spurious
failures.

Problem reported by Graham Reitz on the automake list; see thread:
<http://lists.gnu.org/archive/html/automake/2011-05/msg00022.html>

* automake.in ($traces): Use `@am_AUTOCONF', not simply `autoconf'.
* aclocal.in ($traces): Use `@am_AUTOM4TE', not simply `autom4te'.
* Makefile.am (do_subst): Substitute also `@am_AUTOCONF' and
`@am_AUTOM4TE'.
* NEWS: Update.
* THANKS: Update.