use the libc crate from cargo

Cargo.toml

@@ -9,5 +9,9 @@ gcc = "0.3.3"
 
 [features]
 default = ["os", "valgrind"]
-os = []
+os = ["libc"]
 valgrind = []
+
+[dependencies.libc]
+version = "0.1.6"
+optional = true

src/lib.rs

@@ -3,7 +3,6 @@
 // See the LICENSE file included in this distribution.
 #![feature(no_std)]
 #![feature(asm, core)]
-#![feature(libc)]
 #![no_std]
 
 //! libfringe is a low-level green threading library.