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 ]
configure.ac: Support --disable-maintainer-mode

This

| allows you to choose whether the so called "rebuild rules" should be
| enabled or disabled.  With AM_MAINTAINER_MODE([enable]), they are
| enabled by default, otherwise they are disabled by default.  In the
| latter case, if you have AM_MAINTAINER_MODE in configure.ac, and run
| `./configure && make', then make will *never* attempt to rebuild
| configure, Makefile.ins, Lex or Yacc outputs, etc.  I.e., this
| disables build rules for files that are usually distributed and that
| users should normally not have to update.
|
| The user can override the default setting by passing either
| `--enable-maintainer-mode' or `--disable-maintainer-mode' to
| configure.
|
| People use AM_MAINTAINER_MODE either because they do not want their
| users (or themselves) annoyed by timestamps lossage (see CVS), or
| because they simply can't stand the rebuild rules and prefer running
| maintainer tools explicitly.

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