Refactorings.
* build/virtual-target.jam
(subvariant): Renamed from 'subvariant-dg'.
(subvariant.created-targets, requested-properties, build-properties,
sources-usage-requirements): Renamed some methods and added some new ones.
* build/targets.jam
(basic-target.compute-usage-requirements): Accept 'subvariant' instance.
This allows derived classes to access all kind of information they'd like,
without adding terrible number of parameters.
[SVN r20518]
Vladimir Prus