Print 'warnings are treated as errors' note if needed.

Print 'warnings are treated as errors' note if needed.

* lib/Automake/Channels.pm: Fix typo in comment.
(_print_message): If -Werror is enabled, print a 'warnings are
treated as errors' note before the first such warning.
* tests/warnopts.test: Adjust comment to mention expected warning.
Adjust code to ignore 'warnings are treated as errors' note.
* tests/location.test: Adjust expected output with -Werror.
* tests/werror4.test: New test.
* tests/Makefile.am: Update.
* NEWS: Update.

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