autom4te: always update the output file, even if it hasn’t changed

autom4te: always update the output file, even if it hasn’t changed

Automake generates a Makefile rule for regenerating the configure
script, that relies on an invocation of ‘autoconf’ always bumping the
timestamp on the configure script, even if it hasn’t changed.
The patch to make autom4te update the output file atomically
(1725c947144d9bebfe7817c2c5f0d53d884b1297) broke this.

Fixes several failures in automake’s test suite.

 * bin/autom4te.in (handle_output): Always call update_file with force=1.
 * tests/tools.at (autoconf: timestamp changes): New test.