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

test/library_dirs/subdir/mylib.cc

Index: test/library_dirs/subdir/mylib.cc
===================================================================
--- test/library_dirs/subdir/mylib.cc	(revision 0)
+++ test/library_dirs/subdir/mylib.cc	(revision 0)
@@ -0,0 +1,9 @@
+// Copyright (c) 2013 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 "mylib.h"
+
+std::string my_foo(int x) {
+  return std::string("world");
+}