* bin/autoconf.as: Handle `-' just like other input files.

* bin/autoconf.as: Handle `-' just like other input files.
* bin/autom4te.in (parse_args): Pass `-' through.
(handle_output): Skip the forbidden token search if we read from stdin.
(up_to_date): Always treat stdin as out of date.
* tests/tools.at (autoconf: input from stdin): New test.
(autoconf: forbidden tokens, basic): Check a second `autoconf' run.