Add builder construction variables to Environments before the builder is added.

lib/rscons/application.rb

@@ -15,8 +15,14 @@ module Rscons
     #   Access any variables set on the rscons command-line.
     attr_reader :vars
 
+    # @return [VarSet]
+    #   The default construction variables provided by builders.
+    attr_reader :default_varset
+
+    # Create Application instance.
     def initialize
       @n_threads = determine_n_threads
+      @default_varset = VarSet.new
       @vars = VarSet.new
     end