Vendor dependencies

Let's see how I like this workflow.

vendor/autocfg/examples/integers.rs

@@ -0,0 +1,9 @@
+extern crate autocfg;
+
+fn main() {
+    // Normally, cargo will set `OUT_DIR` for build scripts.
+    let ac = autocfg::AutoCfg::with_dir("target").unwrap();
+    for i in 3..8 {
+        ac.emit_has_type(&format!("i{}", 1 << i));
+    }
+}