Auto merge of #777 - ekmartin:macos_gettime, r=alexcrichton
Auto merge of #777 - ekmartin:macos_gettime, r=alexcrichton

Add clock_gettime related functions to macOS

Fixes https://github.com/rust-lang/libc/issues/776. The constants were retrieved on macOS 10.12.6. Tried adding `clock_nanosleep` too, but it doesn't seem like macOS' libc includes that yet.

cactorium/libc,rust-lang/libc,rust-lang-nursery/libc,cactorium/libc,cactorium/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang/libc,rust-lang-nursery/libc