iOS support

[FeodorFitsner]

A tiny change that makes setuptools-rust work in crossenv environment, originally made by @freakboy3742
It allows to successfully build bcrypt and cryptography for iOS.

[freakboy3742]

FYI - I was deliberately not submitting this PR upstream until such time a I had an opportunity to fully test it with the final state of iOS support in CPython 3.13, and add tests to verify the change.

On a purely functional level, while it successfully builds cryptography, in my testing the final binary doesn't fully work, as there's a runtime load that fails, and I wasn't 100% sure if that would require additional changes to this package.

I don't object to this being merged as is if the maintainers are comfortable with it; at least to my mind, there's more work required before it is ready.

[FeodorFitsner]

Any chance to get this change into the next release of setuptools-rust?

[davidhewitt]

I will be working on the next release of setuptools-rust imminently now that we have 3.13 support in PyO3. So yes, am happy to move forward with this.

Do you know if there's any reasonable way we can test iOS in CI? If not, no problem, let's add a CHANGELOG entry and then merge this as-is 👍

[freakboy3742]

It's definitely possible to build/test packages on iOS; for example here's a CI test run for Toga (a GUI toolkit) running on iOS. You need to use a macOS runner; the compilation process is no slower than it would be for iOS, and the SDKs are all installed and available. Running the code means starting a simulator; that takes 2-3 minutes by itself, so it's not fast, but it's very reliable.

My Q3 TODO list is largely focussed on binary packaging for iOS; I should be in a position to look at this in a couple of weeks. I don't know if that means you want to hold off merging until CI is in place.

[davidhewitt]

Let's merge this for now to unblock you, but avoid declaring full iOS support until CI is in place.