Vendor dependencies

Let's see how I like this workflow.

vendor/autocfg/examples/paths.rs

@@ -0,0 +1,22 @@
+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_path("std::vec::Vec");
+    ac.emit_path_cfg("std::vec::Vec", "has_vec");
+
+    // rustc 1.10.0
+    ac.emit_has_path("std::panic::PanicInfo");
+    ac.emit_path_cfg("std::panic::PanicInfo", "has_panic_info");
+
+    // rustc 1.20.0
+    ac.emit_has_path("std::mem::ManuallyDrop");
+    ac.emit_path_cfg("std::mem::ManuallyDrop", "has_manually_drop");
+
+    // rustc 1.25.0
+    ac.emit_has_path("std::ptr::NonNull");
+    ac.emit_path_cfg("std::ptr::NonNull", "has_non_null");
+}