Adds support for the library_dirs key, which appears in the documentation but was never actually im…

test/mac/library_dirs/subdir/hello.cc

Index: test/mac/library_dirs/subdir/hello.cc
===================================================================
--- test/mac/library_dirs/subdir/hello.cc	(revision 0)
+++ test/mac/library_dirs/subdir/hello.cc	(revision 0)
@@ -0,0 +1,10 @@
+// Copyright (c) 2012 Google Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <iostream>
+
+int main() {
+  std::cout << "Hello, world!" << std::endl;
+  return 0;
+}