Clarify which macOS are supported.

[phadej]

The sad truth is that we can easily support only the few latest macOS version. I.e. 10.15 at the moment.

• GitHub CI: https://help.github.com/en/actions/reference/software-installed-on-github-hosted-runners has macOS 10.15
• Travis CI has bigger list: https://docs.travis-ci.com/user/reference/osx/ but the timelimit on the jobs makes it quite unusable (we were barely able to get cabal-install built and tested there).

Relatedly, it's also not-so-clear which is the oldest macOS supported by GHC official bindists (cc @bgamari). Our CONTRIBUTING.md says that we support cabal-install to be buildable with three years old GHC. So that three-years-oldest GHC's oldest supported macOS sets the ultimate hard-limit.

It would be good to write this down, and see what can be done to actually test on these older configurations. The recent #6833 shows that some people are stuck on them, for whatever reason.

Also local development machines are tricky. I have a 10.12 machine, which I don't upgrade primarily because it could be used to debug stuff. I'm lucky I don't have any other real use for that machine.

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Same applies for Windows, yet Windows 10 is continuously rolling release, which is a curse and a blessing. Should we test with Windows 2016 server edition, as GitHub CI generously offers that option too? Notice however, that the oldest GHC on windows we currently support is GHC-8.6.5 (which is just a one year old). This is very unfortunate.

[phadej]

ping @23Skidoo @hvr

[23Skidoo]

I don't use macOS, so can't comment on the specifics. I believe that Windows is better at backwards compatibility (I would expect a cabal-install binary built with GHC 8.6.5 on Windows 10 today to work on a 5 year old Windows installation).

In general it makes sense to support old OS versions, but only as long as that doesn't require unreasonable effort.

[hasufell]

It would be cool if we can lower the build time on travis with caching, either via cabal-cache and a public S3 bucket or via directory-caching in travis maybe.

[gbaz]

In this case its not just "building" with backwards compat, but I think that cabal is just using an api that isn't necessarily available on older macOS's, so the problem is that some functions in System.Posix aren't available across versions of macOS, and that's not documented anywhere in particular, and hard to trace down without just like... testing on different versions :-/

https://hackage.haskell.org/package/unix-2.7.2.2/docs/src/System.Posix.Files.html#setFileTimesHiRes

(A ticket I hunted down elsewhere (referenced in 6833) suggests that installing some xcode stuff could bring the symbol into scope, but wtf do I know).