missing: don't cater to 'configure.in' in our advice

missing: don't cater to 'configure.in' in our advice

The use of 'configure.in' as autoconf input now deprecated the development
versions of both Automake and Autoconf, and we'd like to speed up its
demise.

* missing: In the advice displayed in case some autotools are detected to
be missing, refer to 'configure.ac' unconditionally, instead of referring
to 'configure.in' when that file exists in the current directory (and no
'configure.ac' does).

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