Auto merge of #1088 - m-hilgendorf:patch-1, r=alexcrichton
Auto merge of #1088 - m-hilgendorf:patch-1, r=alexcrichton

Add ftok on MacOS

Provide a symbol for the `ftok` function declared in sys/ipc.h, for completeness and to allow calling `ftok` to generate the key value for `shmat`.

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