Vendor dependencies

Let's see how I like this workflow.

vendor/cc/tests/cflags.rs

@@ -0,0 +1,15 @@
+mod support;
+
+use crate::support::Test;
+use std::env;
+
+/// This test is in its own module because it modifies the environment and would affect other tests
+/// when run in parallel with them.
+#[test]
+fn gnu_no_warnings_if_cflags() {
+    env::set_var("CFLAGS", "-arbitrary");
+    let test = Test::gnu();
+    test.gcc().file("foo.c").compile("foo");
+
+    test.cmd(0).must_not_have("-Wall").must_not_have("-Wextra");
+}