change "vars" parameters to be required

lib/rscons/builders/program.rb

@@ -14,7 +14,7 @@ module Rscons
       }
     end
 
-    def run(target, sources, cache, env, vars = {})
+    def run(target, sources, cache, env, vars)
       # build sources to linkable objects
       objects = env.build_sources(sources, [env['OBJSUFFIX'], env['LIBSUFFIX']].flatten, cache, vars)
       return false unless objects