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..fdd6f1a682164aeb758b4e707fbfd3aac705e807 100644
--- a/tools/gn/target.h
+++ b/tools/gn/target.h
@@ -89,16 +89,29 @@ class Target : public Item {
   void set_output_name(const std::string& name) { output_name_ = name; }
 
   // Returns the output name for this target, which is the output_name if
-  // specified, or the target label if not. If the flag is set, it will also
-  // include any output prefix specified on the tool (often "lib" on Linux).
+  // specified, or the target label if not.
   //
   // Because this depends on the tool for this target, the toolchain must
   // have been set before calling.
-  std::string GetComputedOutputName(bool include_prefix) const;
+  std::string GetComputedOutputName() 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 +331,9 @@ 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_;