* lib/Automake/Channels.pm (dup_channel_setup, drop_channel_setup): New functions. (@EXPORT): Add them. * automake.in (generate_makefile): Call dup_channel_setup and drop_channel_setup. (process_option_list): Recognize --warnings and -W options. * automake.texi (Options): Document them. * tests/pluseq5.test: Check that -Wno-obsolete will disable the warning. * tests/warnopts.test: New file. * tests/Makefile.am (TESTS): Add warnopts.test.