Bugfix: allow alternative with no sources and no generated targets.
* new/targets.jam
(generate): Use a special string to report failure, not just return
of empty list.
(basic-target.generate-sources): Accomodate the above change.
(basic-target.check-for-unused-sources): Allow empty list of targets.
* test/alternatives.py: New test.
[SVN r17906]
Vladimir Prus