call original method_missing() from Environment's method_missing() when a builder is not found

lib/rscons/environment.rb

@@ -61,11 +61,15 @@ module Rscons
       fname.sub(/\.[^.]*$/, '')
     end
 
+    alias_method :orig_method_missing, :method_missing
+
     def method_missing(method, *args)
       if @builders.has_key?(method.to_s)
         # TODO: build sources if necessary
         builder = @builders[method.to_s].new
         builder.run(self, *args)
+      else
+        orig_method_missing(method, *args)
       end
     end
   end