Vendor dependencies

Let's see how I like this workflow.

vendor/cc/src/bin/gcc-shim.rs

@@ -0,0 +1,48 @@
+#![cfg_attr(test, allow(dead_code))]
+
+use std::env;
+use std::fs::File;
+use std::io::prelude::*;
+use std::path::PathBuf;
+
+fn main() {
+    let mut args = env::args();
+    let program = args.next().expect("Unexpected empty args");
+
+    let out_dir = PathBuf::from(
+        env::var_os("GCCTEST_OUT_DIR").expect(&format!("{}: GCCTEST_OUT_DIR not found", program)),
+    );
+
+    // Find the first nonexistent candidate file to which the program's args can be written.
+    for i in 0.. {
+        let candidate = &out_dir.join(format!("out{}", i));
+
+        // If the file exists, commands have already run. Try again.
+        if candidate.exists() {
+            continue;
+        }
+
+        // Create a file and record the args passed to the command.
+        let mut f = File::create(candidate).expect(&format!(
+            "{}: can't create candidate: {}",
+            program,
+            candidate.to_string_lossy()
+        ));
+        for arg in args {
+            writeln!(f, "{}", arg).expect(&format!(
+                "{}: can't write to candidate: {}",
+                program,
+                candidate.to_string_lossy()
+            ));
+        }
+        break;
+    }
+
+    // Create a file used by some tests.
+    let path = &out_dir.join("libfoo.a");
+    File::create(path).expect(&format!(
+        "{}: can't create libfoo.a: {}",
+        program,
+        path.to_string_lossy()
+    ));
+}