default value will always be present in build properties of all
main targets. The change moves adding default value into main
targets --- it was done at the top level.
* new/build-request.jam
(expand-no-defaults): No longer local.
* new/build-system.jam
Use 'build-request.expand-no-defaults', not 'expand'.
* new/feature.jam
(add-defaults): Tolerate conditional properties
(e.g <variant>debug:<define>DEBUG)
* new/property-set.jam
(property-set.add-defaults): New method.
* new/targets.jam
(basic-target.final-properties): Add defaults.
* test/default_features.py: New test.
[SVN r17753]
