[Boost-build] Toolset requirements

From: Vladimir Prus (ghost_at_[hidden])
Date: 2006-11-15 14:53:55

Attached is a patch to implement 'toolset requirements' -- a mechanism for 
toolset to say "on msvc, with dynamic runtime, use multi-threading". The 
approach is damn simple -- there's global list of requirements that toolset 
modules append to. The list is appended to requirements of all main targets,
as if that list was specified in their requirements.

With this patch, building with msvc starts using threading=multi.

If nobody has any objections to this patch, I'll commit it.

- Volodya