Vendor dependencies

Let's see how I like this workflow.

vendor/core-foundation-sys/build.rs

@@ -0,0 +1,14 @@
+// Copyright 2013-2015 The Servo Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    if std::env::var("TARGET").unwrap().contains("-apple") {
+        println!("cargo:rustc-link-lib=framework=CoreFoundation");
+    }
+}