Clarify how to enable `silent-rules' and other global options. * doc/automake.texi (Options): When introducing options, be more careful to note that not all of them can be specified in AUTOMAKE_OPTIONS statements, and that some do not make sense in subdirectories. Note more prominently that `silent-rules' can only be specified in configure.ac. * lib/Automake/Options.pm (_process_option_list): Improve error message for `silent-rules', `tar-v7', `tar-ustar', `tar-pax' options specified in Makefile.am files. Report by Bruno Haible. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>