Disambiguiate targets and directories passed to 'build-project'.

Previously, if a Jamfile had:
    exe a : a.cpp ;
    build-project a ;
where a is also a subdirectory, we'd get error.


[SVN r61054]

tools/build

@@ -1 +1 @@
-Subproject commit 7bc3446c3c18b07a1bfe3b708c72be4f71184286
+Subproject commit 75de05efec50accd547a95a169ad438ec1bd2593