Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/cargo/build.rs

@@ -0,0 +1,73 @@
+use std::io::{self, Write};
+#[cfg(windows)]
+use std::os::windows::fs as windows;
+use std::path::Path;
+use std::process;
+#[cfg(windows)]
+use std::{env, fs};
+
+const MISSING: &str = "
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When building `cxx` from a git clone, git's symlink support needs
+to be enabled on platforms that have it off by default (Windows).
+Either use:
+
+   $ git config --global core.symlinks true
+
+prior to cloning, or else use:
+
+   $ git clone -c core.symlinks=true https://github.com/dtolnay/cxx
+
+for the clone.
+
+Symlinks are only required when compiling locally from a clone of
+the git repository---they are NOT required when building `cxx` as
+a Cargo-managed (possibly transitive) build dependency downloaded
+through crates.io.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+";
+
+#[cfg(windows)]
+const DENIED: &str = "
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When building `cxx` from a git clone on Windows we need Developer
+Mode enabled for symlink support.
+
+To enable Developer Mode: go under Settings to Update & Security,
+then 'For developers', and turn on the toggle for Developer Mode.
+
+For more explanation of symlinks in Windows, see these resources:
+> https://blogs.windows.com/windowsdeveloper/2016/12/02/symlinks-windows-10/
+> https://docs.microsoft.com/windows/uwp/get-started/enable-your-device-for-development
+
+Symlinks are only required when compiling locally from a clone of
+the git repository---they are NOT required when building `cxx` as
+a Cargo-managed (possibly transitive) build dependency downloaded
+through crates.io.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+";
+
+fn main() {
+    if Path::new("src/syntax/mod.rs").exists() {
+        return;
+    }
+
+    #[allow(unused_mut)]
+    let mut message = MISSING;
+
+    #[cfg(windows)]
+    if let Some(out_dir) = env::var_os("OUT_DIR") {
+        let parent_dir = Path::new(&out_dir).join("symlink");
+        let original_dir = parent_dir.join("original");
+        let link_dir = parent_dir.join("link");
+        if fs::create_dir_all(&original_dir).is_ok()
+            && (!link_dir.exists() || fs::remove_dir(&link_dir).is_ok())
+            && windows::symlink_dir(&original_dir, &link_dir).is_err()
+        {
+            message = DENIED;
+        }
+    }
+
+    let _ = io::stderr().write_all(message.as_bytes());
+    process::exit(1);
+}