Add confstr and guaranteed _CS_* constants on apple

[thomcc]

I actually just want _CS_DARWIN_USER_TEMP_DIR, and the confstr function, but I also have added the other "guaranteed" _CS_* values (e.g. present in the output of man confstr).

This is apparently a POSIX API but I have no idea which other platforms support it, nor what values should be provided. It's somewhat important on Darwin (the only way to access the user temp dir), but I'm not sure what other use it has, so I've only provided it on Darwin.

There are a few other constants in Darwin unistd.h, but they're omitted since they're only conditionally present, and they provide values that... don't actually work¹. As a result, I've only provided the ones documented by confstr documentation.

Footnotes

1. For example, _CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS nominally provides values that can be passed to a c compiler, but it gives -W 64 for me, which does not seem to work with clang. ↩

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) soon.

Please see the contribution instructions for more information.

[JohnTitor]

Thanks! @bors r+

[bors]

📌 Commit 52199a6 has been approved by JohnTitor

It is now in the queue for this repository.

[bors]

⌛ Testing commit 52199a6 with merge f6eb496...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13, checks-cirrus-freebsd-14
Approved by: JohnTitor
Pushing f6eb496 to master...