docs: ACLOCAL_AMFLAGS will become obsolescent in Automake 1.13

docs: ACLOCAL_AMFLAGS will become obsolescent in Automake 1.13

See commit 'v1.12.1-165-gcd1a9cc' "aclocal: deprecate ACLOCAL_AMFLAGS,
trace AC_CONFIG_MACRO_DIR instead" in the Automake git repository.

See also follow-up discussion at:
<http://lists.gnu.org/archive/html/automake-patches/2012-07/msg00010.html>

* doc/autoconf.texi (@node Input @defmac AC_CONFIG_MACRO_DIRS): Adjust.
(autoreconf Invocation): Warn about the possible future removal of
ACLOCAL_AMFLAGS support from Automake.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>