Fix BB54: the 'alias' rule was not passing usage requirements of the sources.

* build/alias.jam
  (alias-target-class.compute-usage-requirements): New method.


[SVN r20519]

tools/build

@@ -1 +1 @@
-Subproject commit 5faa183c0733e47f398bc6d0035a67aa52267203
+Subproject commit 85edc40b252e5428c857203887842140b31428ef