Add newer-api Cargo feature

[JohnTitor]

This is an attempt to accept the breakage-candidates without a major release.

[rustbot]

@JohnTitor: no appropriate reviewer found, use r? to override

[JohnTitor]

I expect this will gate breaking changes (adding members to struct, changing const's values, etc...) but it would add complexity, any opinions or better options are definitely welcome!
cc @rust-lang/crate-maintainers I'd like to hear anyone's opinion.

[Amanieu]

I don't think this will work due to the way Cargo feature unification works. Enabling this feature in one crate could end up breaking a completely unrelated crate. Considering how widely used libc is, I think this will be too brittle for any practical use.

[JohnTitor]

Hmm, I see! Do you have any other ways to accept breakage candidates until we can release libc 0.3? 🤔 (Or, would releasing libc 0.3 be the best way here?)

[Amanieu]

I think the best way forward would be to follow the implementation plan in #3248.

[JohnTitor]

Sure, let's follow it then!