Add a `--clean' option for autoconf, autoheader, autoreconf and autom4te.
* bin/autoconf.as, bin/autoheader.in, bin/autoreconf.in,
bin/autom4te.in: New option `--clean'.
* NEWS: Mention it.
* bin/autoreconf.in (&run_make): New.
(&autoreconf_current_directory): Use it. Honor the $clean variable.
* lib/Autom4te/General.pm ($clean): New boolean value.
(getopt): Handle `--clean'.
* lib/Autom4te/FileUtils.pm ($SIMPLE_BACKUP_SUFFIX): Export it.
* tests/torture.at: Test the new feature.
* doc/autoconf.texi (autoconf Invocation, autoreconf Invocation,
autoheader Invocation, autom4te Invocation): Document the new option.
Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr>