src/Pure/System/build.scala

   def build(all_sessions: Boolean, build_images: Boolean, list_only: Boolean,
     more_dirs: List[Path], options: List[String], sessions: List[String]): Int =
   {
     val full_queue = find_sessions(more_dirs)
 
+    val build_options = (Options.init() /: options)(_.define_simple(_))
+
     sessions.filter(name => !full_queue.defined(name)) match {
       case Nil =>
       case bad => error("Undefined session(s): " + commas_quote(bad))
     }