Add GN output prefix override and allow empty output extensions.

tools/gn/substitution_writer.cc

 // Copyright 2014 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/substitution_writer.h"
 
 #include "tools/gn/build_settings.h"
 #include "tools/gn/escape.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/output_file.h"
@@ skipping 529 matching lines @@
     const Tool* tool,
     SubstitutionType type) {
   // First try the common tool ones.
   std::string result;
   if (GetTargetSubstitution(target, type, &result))
     return result;
 
   // Fall-through to the linker-specific ones.
   switch (type) {
     case SUBSTITUTION_OUTPUT_EXTENSION:
-      // Use the extension provided on the target if nonempty, otherwise
+      // Use the extension provided on the target if specified, otherwise
       // fall back on the default. Note that the target's output extension
       // does not include the dot but the tool's does.
+      if (!target->output_extension_set())
+        return tool->default_output_extension();
       if (target->output_extension().empty())
-        return tool->default_output_extension();
+        return std::string();  // Explicitly set to no extension.
       return std::string(".") + target->output_extension();
 
     default:
       NOTREACHED();
       return std::string();
   }
 }