Preserve old encinit.c if unchanged

enc/encinit.c.erb

@@ -1,3 +1,6 @@
+/* Automatically generated from <%= erb.filename %>
+ * Do not edit<%# directly%>.
+ */
 /* Copyright 2012 Google Inc. Some Rights Reserved.
  * Author: yugui@google.com (Yugui Sonoda)
  */

enc/make_encmake.rb

@@ -147,10 +147,6 @@ if MODULE_TYPE == :static
     Dir.mkdir 'enc'
   rescue Errno::EEXIST
   end
-  File.open("enc/encinit.c", "w") {|f|
-    f.puts "/* Automatically generated from enc/encinit.c.erb"
-    f.puts " * Do not edit."
-    f.puts " */"
-    f.puts tmp
-  }
+  require 'tool/lib/output'
+  Output.new(path: "enc/encinit.c", ifchange: true).write(tmp)
 end