create Builder instance for each build operation - close #88

build_tests/custom_builder/Rsconscript

@@ -11,7 +11,7 @@ end
 
 build do
   Environment.new do |env|
-    env.add_builder(MySource.new)
+    env.add_builder(MySource)
     env.MySource('inc.h', [])
     env.Program('program.exe', Dir['*.c'])
   end