ignore more vim swap files
populate API to prove XS in Makefile.PL/Build.PL

Idea behind is:

check_produce_xs_build ( \@langs || [C] ) {
       ! check_pureperl_required
               and
       check_valid_compilers( $_[0] || [C] ) (aka check_prog_compilers)
               and
       check_valid_perlapi_or_die
}

check_produce_loadable_xs_build ( \@langs || [C] ) {
       check_produce_xs_build ( @_ )
               and
       (
               $ENV{PERL5_AC_OPTS} doesn't contain notest_loadable_xs
                       and
               check_linkable_xs_so_or_die
                       and
               check_loadable_xs_so_or_die
       )
}