Make extconf.rb honor AR and LD variables in addition to CC

[ziggythehamster]

What problem is this PR intended to solve?

#3160

Note I also have branches that fix this on v1.16.x and v1.15.x, but I wanted to make sure I actioned feedback in one place before I opened additional MRs.

Also note that Nokogiri doesn't seem to honor variables passed as gem configuration args, and I didn't add that here because I'm not entirely sure what the "canonical" way to do this is with mkmf/mini_portile. In other situations where I need to provide the toolchain (rdkafka/karafka-rdkafka), there seems to be no special code to convert FOO=BAR to an env var.

That said, it's not actually necessary in my case because using AR and LD from RbConfig is sufficient. So one might say if you need to do something different from that, maybe it's actually a good thing that you would need to do AR=x gem install nokogiri rather than gem install nokogiri -- AR=x. I can take a stab at making this work, though, if the team thinks that both ways should work.

Have you included adequate test coverage?

N/A but I assume if it breaks the build in some configuration I didn't test, GitHub Actions will be mad about it :).

Does this change affect the behavior of either the C or the Java implementations?

No

[ziggythehamster]

Also, let me know what the policy is w/r/t updating the CHANGELOG.md.

[flavorjones]

Thanks for opening this. I'm on holiday for a few more days and will review as soon as I'm back.

One note, though: the 1.15 branch is unsupported and I don't want to cut any more releases on that series. I hope that's not an unpleasant surprise.

[ziggythehamster]

Meh, I suppose I have no choice but to live with it :).

[flavorjones]

I'll take care of the changelog entry once this merges. Thank you for contributing this!

Edit: changelog commit is 76c4373

[flavorjones]

(And if you really need a patch release for v1.16.x I'll do it, just please open a PR on the v1.16.x branch to let me know.)