Vendor dependencies

Let's see how I like this workflow.

vendor/windows_x86_64_gnullvm/build.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let target = std::env::var("TARGET").unwrap();
+    if target != "x86_64-pc-windows-gnullvm" {
+        return;
+    }
+
+    let dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
+
+    println!("cargo:rustc-link-search=native={}", std::path::Path::new(&dir).join("lib").display());
+}