gn: Support build directories outside the source tree.

tools/gn/ninja_toolchain_writer.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/ninja_toolchain_writer.h"
 
 #include <fstream>
 
 #include "base/files/file_util.h"
 #include "base/strings/stringize_macros.h"
@@ skipping 14 matching lines @@
 
 NinjaToolchainWriter::NinjaToolchainWriter(
     const Settings* settings,
     const Toolchain* toolchain,
     const std::vector<const Target*>& targets,
     std::ostream& out)
     : settings_(settings),
       toolchain_(toolchain),
       targets_(targets),
       out_(out),
-      path_output_(settings_->build_settings()->build_dir(), ESCAPE_NINJA) {
+      path_output_(settings_->build_settings()->build_dir(),
+                   settings_->build_settings()->root_path_utf8(),
+                   ESCAPE_NINJA) {
 }
 
 NinjaToolchainWriter::~NinjaToolchainWriter() {
 }
 
 void NinjaToolchainWriter::Run() {
   WriteRules();
   WriteSubninjas();
 }
 
@@ skipping 82 matching lines @@
   // Write subninja commands for each generated target.
   for (size_t i = 0; i < targets_.size(); i++) {
     OutputFile ninja_file(targets_[i]->settings()->build_settings(),
                           GetNinjaFileForTarget(targets_[i]));
     out_ << "subninja ";
     path_output_.WriteFile(out_, ninja_file);
     out_ << std::endl;
   }
   out_ << std::endl;
 }