Handle multiple suffix rules with the same input extension.

Handle multiple suffix rules with the same input extension.
For PR automake/37.

* automake.in (suffix_rules_default): New variable.
(suffix_rules): Redefine as a hash of hashes of pairs.
(initialize_per_input): Setup suffix_rules_default from
suffix_rules on first call, an override suffix_rules
with suffix_rules_default on following calls.
(struct) <output_extensions>: New attribute.
(register_language): Set the default for output_extensions.
Call register_suffix_rule for each suffix.
(derive_suffix, handle_languages): Adjust to the new definition
of $suffix.
(register_suffix_rule): New function.
(rule_define): Call register_suffix_rule.
* tests/suffix8.test, tests/suffix9.test, tests/suffix10.test:
New files.
* tests/Makefile.am (TESTS): Add them.