Rename configure.in to configure.ac

The old name has been deprecated years ago.  The Autoconf documentation
says:

| 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.

[ https://www.gnu.org/software/autoconf/manual/autoconf.html ]