external libc crate triggers unstable library feature warnings

[brson]

I have a cargo project that links to libc 0.1.1. I see this warning

src/lib.rs:4:1: 4:19 warning: use of unstable library feature 'libc'
src/lib.rs:4 extern crate libc;
             ^~~~~~~~~~~~~~~~~~
src/lib.rs:4:1: 4:19 help: add #![feature(libc)] to the crate attributes to silence this warning
src/lib.rs:4 extern crate libc;
             ^~~~~~~~~~~~~~~~~~

This shouldn't happen with the out-of-tree crate.

[alexcrichton]

Dupe of #21771

[alexcrichton]

(fixed by #21782)