Fix per-Makefile.am setting of -Werror.

Fix per-Makefile.am setting of -Werror.

Before this patch, 'AUTOMAKE_OPTIONS = -Werror' in one
Makefile.am would carry over to other Makefile.am files
treated afterwards by the same thread, causing inconsistent
and unstable exit status values.

* lib/Automake/Channels.pm (dup_channel_setup)
(drop_channel_setup): Save and restore the setting of
$warnings_are_errors.
* tests/werror3.test: New test.
* tests/Makefile.am: Adjust.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>