Simplify target paths.

* build/targets.jam
  (project-target.reference-properties): Remove.

* build/virtual-target.jam
  (abstract-file-target.specific-main-target,
   file-target.specific-main-target): Remove.


[SVN r20516]

tools/build

@@ -1 +1 @@
-Subproject commit fcc25341e009b18f366b044eaf2cf495109fe58b
+Subproject commit 75df6c10471a7dc60505e98393a04982524e4551