First implementation of "stage" and "prebuilt" rule.
* new/common.jam (copy): New rule.
* new/targets.jam (main-target.generate): Use a different
algorith for selecting subvariant. Favour one with
the longest intersection of requirements with build
properties.
* new/virtual-target.jam (virtual-target): Eliminate 'subvariant'
attribute. Use properties of action to for the same
purpose. New methods 'set-path' and 'extra-grist'.
[SVN r16129]
Vladimir Prus