Add GN output prefix override and allow empty output extensions.

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 836 matching lines @@
           lib_escape_opts);
     } else {
       out_ << " " << tool_->lib_switch();
       EscapeStringToStream(out_, lib_value, lib_escape_opts);
     }
   }
   out_ << std::endl;
 }
 
 void NinjaBinaryTargetWriter::WriteOutputExtension() {
-  out_ << "  output_extension = ";
-  if (target_->output_extension().empty()) {
-    // Use the default from the tool.
-    out_ << tool_->default_output_extension();
-  } else {
-    // Use the one specified in the target. Note that the one in the target
-    // does not include the leading dot, so add that.
-    out_ << "." << target_->output_extension();
-  }
+  out_ << "  output_extension = "
+       << SubstitutionWriter::GetLinkerSubstitution(
+              target_, tool_, SUBSTITUTION_OUTPUT_EXTENSION);
   out_ << std::endl;
 }
 
 void NinjaBinaryTargetWriter::WriteSolibs(
     const std::vector<OutputFile>& solibs) {
   if (solibs.empty())
     return;
 
   out_ << "  solibs =";
   path_output_.WriteFiles(out_, solibs);
@@ skipping 128 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;
 }