suffix rules: better distinction between builtin and user-derived

suffix rules: better distinction between builtin and user-derived

Fixes automake bug#14441.

* lib/Automake/Rule.pm ($_suffix_rules_default): Remove, superseded by ...
(%suffix_rules_builtin): ... this lexical variable.
($suffix_rules): Remove, superseded by ...
(%suffix_rules): ... this lexical variable.
(suffix_rules, suffix_rules_count): Adjust.
(register_suffix_rule): Update '%suffix_rules_builtin' rather than
'%suffix_rules' if the location (as passed by the '$where' argument)
is an "internal" one (doesn't come from user-provided Makefile.am).
(reset): Simplify resetting of '%suffix_rules' to the default ones
accordingly.
* t/list-of-tests.mk (XFAIL_TESTS): Drop test 'suffix-custom-pr14441.sh'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>