Builds fail on old macOS versions that have `libtool` versions w/o arg file support

[rrbutani]

We use libtool instead of llvm-ar on macOS because the commands that Bazel generates on macOS hosts don't work with llvm-ar.

I don't know when support for arg files (i.e. libtool @arg-file) was added to macOS's libtool but at the very least macOS 10.12's libtool (cctools-900) does not support this.

Bazel's native rules_cc machinery does not seem to pick up on this so builds fail on such machines at the archiving step.

[siddharthab]

Just curious. Where is macOS 10.12 being used? Its last security update was 2 years ago.

[rrbutani]

Oh sorry, I probably should have mentioned; this is just an old machine that I have. I think it's completely valid to decide that this isn't something bazel-toolchain will support.

---

Looking through the releases here, it looks like 949.0.1 is the oldest listed release with arg file support which I think isn't that old (a couple of years or so). I'm not sure how cctools versions map to macOS releases.

[keith]

They map to Xcode releases. I believe I checked when I made this change in bazel and thought the latest 3 major versions supported it, but I don't recall. It's not super clear what bazel's support for those versions are, especially since Apple also restricts what versions are required for shipping apps in the App Store.

[siddharthab]

It's alright. And the PR you sent also looks OK.

I think in general, we should not invite structural/organizational complexity into the codebase to support older releases of stuff. This project is pre-release, and its intended target audience would usually stay current with releases. I generally prefer code simplicity over supporting a very minor use case.

I think it is OK to drop support for:

1. old versions of LLVM, say more than 3 or 4 versions behind the current release.
2. Linux and macOS versions that are past their end-of-life.
3. non-LTS releases of bazel.