Warn on unused sources.

* new/targets.jam
    (basic-target.check-for-unused-sources): New rule.
    (basic-target.generate): Call the above.

* new/virtual-target.jam
    (traverse): New arguments 'include-roots' and 'include-sources'.

* test/unused.py: New test.


[SVN r17685]

tools/build

@@ -1 +1 @@
-Subproject commit 4e531ecc652677cb1f34a4de8da5d787c98185fe
+Subproject commit 5decdacc6be859af91cb608fb5d9278abbd020b0