Fix a link failure on Unix for a main -> libb -> liba scenario,

where libb is a target in Boost.Build and liba is a file referred to by name.


[SVN r30590]

tools/build

@@ -1 +1 @@
-Subproject commit 3e3abd45095b522d7012206e049c890e89aaeeaa
+Subproject commit db0d944e9403e5a11fd6e2f5c949be796294a476