lang, suffix rules: don't require C stuff needlessly

lang, suffix rules: don't require C stuff needlessly

This change fixes automake bug#14560: when two or more user-defined suffix
rules were present in a single Makefile.am, automake would needlessly
include definition of some make variables related to C compilation in the
generated Makefile.in.

* automake.in (handle_languages): Fix logic to decide whether or not to
include definitions of C compilation related variables in the generated
Makefile.in: instead of doing so when two or more user-defined suffix
rules are seen (which is a completely bogus criterion), do so when two
or more compiled languages are used.
* lib/Automake/Rule.pm (suffix_rules_count): Remove as no longer used.
(@EXPORT): Adjust.
* t/list-of-tests.mk (XFAIL_TESTS): No longer list the test script
'suffix-extra-c-stuff-pr14560.sh', which now passes.
* NEWS: Update.

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