macOS deployment targets

[waywardmonkeys]

There's discussion elsewhere about changing the minimum macOS / iOS versions supported: rust-lang/rust#104385

There's Travis CI configs here that used to test this sort of thing for these crates.

I've opened #623 to remove those configs and this was also mentioned in #461.

What should the minimum targets be? How tested is this? Can we test with MACOSX_DEPLOYMENT_TARGET and modern Xcode? This appears to be what the main Rust CI is doing? Or is that only for include processing and not linking?

In any event, it would be good to get this clearly documented as well.

[madsmtm]

I'm somewhat working on doing version detection in madsmtm/objc2#266, just for reference.

If you want something that works now, then checking MACOSX_DEPLOYMENT_TARGET in build.rs is probably a viable solution

[thomcc]

rustc --print deployment-target works as of 1.71.0 (rust-lang/rust#105354)