passing compiler flags to boost libraries (such as Thread) which require compilation

To specify what architecture to compile for, specify the architecture feature when invoking b2.

To specify compiler options that don't already have built-in features, specify the cxxflags feature when invoking b2.

To specify linker options that don't already have built-in features, specify the linkflags feature when invoking b2.

All of these are listed in the Boost.Build docs.