Get rid of annoying 'unused source' warning for stage targets.

* new/stage.jam
  (stage-target-class.check-for-unused-sources): Override base method with
  an empty one, since the inherited logic does not make any sense for stage.


[SVN r18295]

tools/build

@@ -1 +1 @@
-Subproject commit bacc81dfc1f7a0f008bebd04f304718842f4fb81
+Subproject commit e540ddb202a30235feaded26f811fa93eb22cf0b