[boost] Build changes for 1.40

FYI, I've merged Boost.Build from trunk to release branch. The most important change is that now, on Unix, default build with act as if --layout=system is specified, creating libraries with no decoration that can be conveniently used. Other changes are documented at: http://beta.boost.org/users/news/version_1_40_0 Let me know if this causes any problems. - Volodya