Do not compile the C extension on TruffleRuby

[eregon]

• Before this it was compiled but not used, because TruffleRuby has a stringio.rb in stdlib and .rb has precedence over .so. In fact that extension never worked on TruffleRuby, because rb_io_extract_modeenc() has never been defined on TruffleRuby.
• So this just skips compiling the extension since compilation of it now fails: https://github.com/ruby/openssl/issues/699

The approach is similar to the one in strscan, notably:

Write conditions in a way that any other Ruby implementation would simply use its stdlib until it is added explicit support in this gem.

TruffleRuby is not added in CI because there are quite a few failures:

89 tests, 508 assertions, 22 failures, 7 errors, 0 pendings, 1 omissions, 0 notifications
67.0455% passed

It makes more sense to fix https://github.com/oracle/truffleruby/blob/master/lib/truffle/stringio.rb to pass more of the tests before adding TruffleRuby to the CI.

Note that gem install stringio and any Gemfile transitively including stringio is broken on truffleruby-dev until this is merged and released. So it would be awesome if we could have a quick release of stringio with this fix.

[eregon]

@kou All done now.

[eregon]

@nobu Thanks for merging and the changelog entry! Could you make a release or should @kou do that?

[kou]

Released!

[junaruga]

Thank you for releasing the stringio 3.1.0! https://rubygems.org/gems/stringio

[eregon]

Thanks for the release. For some reason it seems RubyGems & Bundler can't use 3.1.0 yet, maybe some caching issue? I filed https://github.com/rubygems/rubygems/issues/7202