Cleanup main target alternative creationg.
* new/targets.jam
(abstract-target.name): New method.
(abstract-target.project): New method.
(main-target-alternative): Simplify. Only register
main target alternative. Don't to fancy processing
of arguments.
(main-target-requirements): New rule.
(main-target-use-requirements): New rule.
(main-target-default-build): New rule.
[SVN r16249]
Vladimir Prus