There are only a few diffs to resolve in the JRuby version, other than being a bit behind:

• Windows check cannot use RUBY_PLATFORM on JRuby, so it uses RbConfig as well now.
• Using IPAddr for comparing addresses. I'm not sure if this is necessary anymore (tests pass without it, but unsure how well this is tested). See Use ipaddr to compare the sender's address jruby/jruby#4496.
• A leak fix that does not seem to have been ported to ruby/resolve, but the leak may have been fixed in another way. See use same parameter for DNS.allocate_request_id/free_request_id to fix leak jruby/jruby#5722.
• A leak fix that does seem to have been ported, since the lazy_initialize logic exists in both versions. See backport fd leak in resolv jruby/jruby#5524.

With only the Windows check fixed, tests pass on JRuby (double-confirmed by replacing JRuby's local copy with the edited version from this repo). I need some assistance from other maintainers as to whether these three PRs need to be ported over. cc @hsbt @nobu

Fixes #19. Once merged and released, JRuby can start shipping resolv as a default gem, rather than a non-gem stdlib.