create Builder instance for each build operation - close #88

build_tests/typical/backward_compatible_build_hooks.rb

@@ -1,12 +1,12 @@
 class MyObject < Rscons::Builder
   def run(target, sources, cache, env, vars)
-    env.run_builder(env.builders["Object"], target, sources, cache, vars)
+    env.run_builder(env.builders["Object"].new, target, sources, cache, vars)
   end
 end
 
 build do
   Environment.new(echo: :command) do |env|
-    env.add_builder(MyObject.new)
+    env.add_builder(MyObject)
     env.append('CPPPATH' => Rscons.glob('src/**'))
     env.add_build_hook do |build_op|
       if build_op[:builder].name == "MyObject" && build_op[:sources].first =~ %r{one\.c}

build_tests/typical/echo_command_string.rb

@@ -9,7 +9,7 @@ end
 build do
   Environment.new do |env|
     env.echo = :command
-    env.add_builder(MyBuilder.new)
+    env.add_builder(MyBuilder)
     env.MyBuilder("foo")
   end
 end