interruptible dependencies, which can take a few seconds;

--- a/src/Pure/Tools/build.scala	Wed Apr 23 12:39:23 2014 +0200
+++ b/src/Pure/Tools/build.scala	Wed Apr 23 12:51:55 2014 +0200
@@ -423,6 +423,8 @@
       verbose: Boolean, list_files: Boolean, tree: Session_Tree): Deps =
     Deps((Map.empty[String, Session_Content] /: tree.topological_order)(
       { case (deps, (name, info)) =>
+          if (progress.stopped) throw Exn.Interrupt()
+
           try {
             val (preloaded, parent_syntax) =
               info.parent match {