Add an output_dir override to GN.

tools/gn/ninja_binary_target_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_binary_target_writer.h"
 
 #include <stddef.h>
 #include <string.h>
 
 #include <cstring>
@@ skipping 762 matching lines @@
   // End of the link "build" line.
   out_ << std::endl;
 
   // The remaining things go in the inner scope of the link line.
   if (target_->output_type() == Target::EXECUTABLE ||
       target_->output_type() == Target::SHARED_LIBRARY ||
       target_->output_type() == Target::LOADABLE_MODULE) {
     WriteLinkerFlags(optional_def_file);
     WriteLibs();
   }
-  WriteOutputExtension();
+  WriteOutputSubstitutions();
   WriteSolibs(solibs);
 }
 
 void NinjaBinaryTargetWriter::WriteLinkerFlags(
     const SourceFile* optional_def_file) {
   out_ << "  ldflags =";
 
   // First the ldflags from the target and its config.
   EscapeOptions flag_options = GetFlagOptions();
   RecursiveTargetConfigStringsToStream(target_, &ConfigValues::ldflags,
@@ skipping 46 matching lines @@
           out_, lib_value.substr(0, lib_value.size() - framework_ending.size()),
           lib_escape_opts);
     } else {
       out_ << " " << tool_->lib_switch();
       EscapeStringToStream(out_, lib_value, lib_escape_opts);
     }
   }
   out_ << std::endl;
 }
 
-void NinjaBinaryTargetWriter::WriteOutputExtension() {
+void NinjaBinaryTargetWriter::WriteOutputSubstitutions() {
   out_ << "  output_extension = "
        << SubstitutionWriter::GetLinkerSubstitution(
               target_, tool_, SUBSTITUTION_OUTPUT_EXTENSION);
   out_ << std::endl;
+  out_ << "  output_dir = "
+       << SubstitutionWriter::GetLinkerSubstitution(
+              target_, tool_, SUBSTITUTION_OUTPUT_DIR);
+  out_ << std::endl;
 }
 
 void NinjaBinaryTargetWriter::WriteSolibs(
     const std::vector<OutputFile>& solibs) {
   if (solibs.empty())
     return;
 
   out_ << "  solibs =";
   path_output_.WriteFiles(out_, solibs);
   out_ << std::endl;
@@ skipping 127 matching lines @@
           "\n"
           "In the latter case, either rename one of the files or move one of\n"
           "the sources to a separate source_set to avoid them both being in\n"
           "the same target.");
       g_scheduler->FailWithError(err);
       return false;
     }
   }
   return true;
 }