cleanup: don't support configure.in anymore

cleanup: don't support configure.in anymore

The autoconf input should be named 'configure.ac' instead.  The use
of 'configure.in' has been deprecated in Autoconf since at least
the 2.13 -> 2.50 transition, and future Autoconf versions (starting
with 2.70 probably) will start to warn about it at runtime.

* lib/Automake/Configure_ac.pm (find_configure_ac): Don't look for
configure.in if configure.ac is not found.
(require_configure_ac): On failure, don't tell that "'configure.ac'
or 'configure.in' is required", but just that "'configure.ac' is
required".
* automake.in (@common_sometimes): Don't list 'configure.in'.
(scan_autoconf_files): Adjust heading comments.
* NG-NEWS: Update.
* t/help.sh: Adjust.
* t/configure.sh: Adjust as obsolete.

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