maint: deprecate 'configure.in' as autoconf input

maint: deprecate 'configure.in' as autoconf input

It has been years since that has been deprecated in the documentation,
in favour of 'configure.ac':

    Previous versions of Autoconf promoted the name configure.in, which
    is somewhat ambiguous (the tool needed to process this file is not
    described by its extension), and introduces a slight confusion with
    config.h.in and so on (for which '.in' means "to be processed by
    configure"). Using configure.ac is now preferred.

It's now time to start giving runtime warning about the use of
'configure.in', so that support for it can be removed in future
versions of autoconf/automake.

See also, in the Autoconf repository, commit 'v2.69-4-g560f16b' of
2012-05-23, "general: deprecate 'configure.in' as autoconf input".

* lib/Automake/Configure_ac.pm: Issue a warning in the 'obsolete'
category if 'configure.in' is detected.  Since this module is synced
from Automake, this change is to be backported there (and will be
soon).
* t/help.sh: Adjust.
* t/configure.sh: Adjust and enhance.

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