Drop support for 'configure.in' as the Autoconf input file

Drop support for 'configure.in' as the Autoconf input file

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.  Automake
has been warning about it since the 1.13 release.

This change is basically a backport of commit 'v1.12-325-g927d4cb',
from Automake-NG.

* 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.
* NEWS: Update.
* t/help.sh: Adjust.
* t/configure.sh: Remove as obsolete.
* t/list-of-tests.mk: Adjust.