gn: Support build directories outside the source tree.

tools/gn/item.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/item.h"
 
 #include "base/logging.h"
+#include "tools/gn/settings.h"
 
 Item::Item(const Settings* settings, const Label& label)
     : settings_(settings),
       label_(label),
       defined_from_(NULL) {
 }
 
 Item::~Item() {
 }
 
@@ skipping 11 matching lines @@
     return "target";
   if (AsToolchain())
     return "toolchain";
   NOTREACHED();
   return "this thing that I have no idea what it is";
 }
 
 bool Item::OnResolved(Err* err) {
   return true;
 }