Warnings win over strictness in AUTOMAKE_OPTIONS.

Warnings win over strictness in AUTOMAKE_OPTIONS.

Ensure that, for what concerns the options specified in
AUTOMAKE_OPTIONS, explicitly-defined warnings always take
precedence over implicit strictness-implied warnings.

This finally fixes Automake bug#7669 a.k.a. PR/547.

* automake.in (handle_options): Call 'process_option_list'
only once per set of options.
* lib/Automake/Options.pm (process_global_option_list,
process_option_list): Add sanity checks.
($_options_processed, $_global_options_processed): New
internal variables, used by the sanity checks above.
* tests/warnings-win-over-strictness.test: Extend.