* bin/autoconf.in (--include, -I): New option.

* bin/autoconf.in (--include, -I): New option.
Map --localdir, --autoconf-dir onto it.
Forward autom4te's options instead of interpreting them.
* bin/autoconf.in, bin/autoheader.in (AC_MACRODIR, autoconf_dir):
There is no such envvar since the inception of autom4te.cfg.
* bin/autom4te.in (&parse_args): Uniquify `@include'.
* bin/autoupdate.in: Adjust, and perform more control.
* tests/atspecific.m4 (AT_CHECK_AUTOCONF): Adjust.
* tests/autoconf: Dittowise.