Bump x86_64 macOS build to use El Capitan

[Bo98]

Homebrew 3.5.0 is targetted to drop Yosemite support, so let's drop it here too.

I've set up a temporary CI node to test a 10.11 cross-compile on 10.13 (using 10.11 dev tools: Xcode 7.3.1). Very much an experimental setup but we'll see how it works.

CI changes will be dropped from the PR and I'll do it properly in a subsequent PR.

[Bo98]

10.11 cross-compile on 10.13 (using 10.11 dev tools: Xcode 7.3.1)

So the way I've done this is very much like a cross-compile that people would do for cross-archs or cross-systems. It sounds like overkill for doing so across macOS versions, but I think it is significantly safer to do so as it should disable any runtime configure checks that base things on the current running OS.

One caveat of cross-compiling is that Ruby requires the host Ruby to be the same version, which it isn't here. Instead of compiling twice, I have put in a workaround to use the in-tree miniruby like the native build would. It's definitely a bit hacky but at least it doesn't require any inreplace calls.

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OpenSSL build currently fails for unrelated reasons. Looks like a test certificate has expired the moment we've entered June. It affects upstream's CI too so there should be a patch there soon. Once that's fixed we should have a test bottle we can try.

[MikeMcQuaid]

@Bo98 Nice work here so far. Just FYI: I think given the amount of code changes we're requiring for this: unless this works with minimal/no more code changes needed by EOD today or tomorrow: we should just raise the minimum requirement to 10.12 instead.

[Bo98]

I think this is it - I don't plan any more changes. OpenSSL change is unrelated, so the only code changes are:

• Passing --build and --host to enable cross-compiling for the formulae using autotools configure.
• A small hack for Ruby as described above to avoid needing to compile twice.

Here's a bottle build you can try: https://github.com/Homebrew/homebrew-portable-ruby/suites/6748677066/artifacts/257816018. It's not the release build since it doesn't have the revision bump but it's otherwise complete I think.

[Bo98]

brew tests passes with this on El Capitan, except some Cask quarantine tests, which are definitely a recent-ish regression but not likely to be Ruby related. So it looks like our cross-compile setup works!

I'm also going to run it against Ruby's test suite (or at least the parts which can run against an external Ruby install) but I reckon anything it flags there will be pre-existing.

[MikeMcQuaid]

Great work @Bo98!