attribute which controls where generated targets are put,
much in the same was as ALL_LOCATE_TARGET.
* new/virtual-target.jam
(virtual-target.path): Respect build dir.
(virtual-target.actualize-location): Don't
compute path, but call call instead.
* new/project.jam
(initialize): Set default value for 'build-dir'.
(project-attributes.set): Handle 'build-dir'.
* new/path.jam
(relative): New rule.
* test/build_dir.py: New test.
[SVN r16589]