news: support for configure.in will be dropped in future automake versions

news: support for configure.in will be dropped in future automake versions

The use of configure.in as Autoconf input has been deprecated for a
very long time in the Autoconf documentation, and the next version of
Autoconf (2.70) will start warning about it ar runtime as well (see
commit 'v2.69-4-g560f16b' or 2012-05-23, "general: deprecate
'configure.in' as autoconf input", in the Autoconf's git repository).

* NEWS (Future backward-incompatibilities): Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>