Clarify how to enable `silent-rules' and other global options.

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>