Add GN output prefix override and allow empty output extensions.

tools/gn/target.h

Index: tools/gn/target.h
diff --git a/tools/gn/target.h b/tools/gn/target.h
index c3fed0122523b96d60805f00aa211bb5d2521d38..30ce39142482734475a991c45a090bd3840219b1 100644
--- a/tools/gn/target.h
+++ b/tools/gn/target.h
@@ -96,9 +96,23 @@ class Target : public Item {
   // have been set before calling.
   std::string GetComputedOutputName(bool include_prefix) const;
 
+  bool output_prefix_override() const { return output_prefix_override_; }
+  void set_output_prefix_override(bool prefix_override) {
+    output_prefix_override_ = prefix_override;
+  }
+
+  // The output extension is really a tri-state: unset (output_extension_set
+  // is false and the string is empty, meaning the default extension should be
+  // used), the output extension is set but empty (output should have no
+  // extension) and the output extension is set but nonempty (use the given
+  // extension).
   const std::string& output_extension() const { return output_extension_; }
   void set_output_extension(const std::string& extension) {
     output_extension_ = extension;
+    output_extension_set_ = true;
+  }
+  bool output_extension_set() const {
+    return output_extension_set_;
   }
 
   const FileList& sources() const { return sources_; }
@@ -318,7 +332,9 @@ class Target : public Item {
 
   OutputType output_type_;
   std::string output_name_;
+  bool output_prefix_override_;
   std::string output_extension_;
+  bool output_extension_set_;
 
   FileList sources_;
   bool all_headers_public_;