not by project attribute. The problem with the previous interface was
that it was not possible to mark target as explicit anywhere else,
such as in helper module which declares target in project module, or
in toolset module.
* new/targets.jam
(project-target.mark-target-as-explicit): New rule
(project-target.targets-to-build): Use self.explicit-targets.
* new/project.jam
(project-attributes.set): Don't allow 'explicit-targets'.
[SVN r18900]
