Vendor dependencies

Let's see how I like this workflow.

vendor/autocfg/examples/traits.rs

@@ -0,0 +1,26 @@
+extern crate autocfg;
+
+fn main() {
+    // Normally, cargo will set `OUT_DIR` for build scripts.
+    let ac = autocfg::AutoCfg::with_dir("target").unwrap();
+
+    // since ancient times...
+    ac.emit_has_trait("std::ops::Add");
+    ac.emit_trait_cfg("std::ops::Add", "has_ops");
+
+    // trait parameters have to be provided
+    ac.emit_has_trait("std::borrow::Borrow<str>");
+    ac.emit_trait_cfg("std::borrow::Borrow<str>", "has_borrow");
+
+    // rustc 1.8.0
+    ac.emit_has_trait("std::ops::AddAssign");
+    ac.emit_trait_cfg("std::ops::AddAssign", "has_assign_ops");
+
+    // rustc 1.12.0
+    ac.emit_has_trait("std::iter::Sum");
+    ac.emit_trait_cfg("std::iter::Sum", "has_sum");
+
+    // rustc 1.28.0
+    ac.emit_has_trait("std::alloc::GlobalAlloc");
+    ac.emit_trait_cfg("std::alloc::GlobalAlloc", "has_global_alloc");
+}