[boost] [B2] Propagate CXX requirements to consumers

Hello, I'm trying to specify that my library uses C++11 (features) and cannot be compiled or used without. Using `[ requires cxx11_defaulted_functions]` as the project "requirements" does work (lib not build), but e.g. the tests are still build and obviously failing. Putting the above as "usage-requirements"  (just like `<link>shared:<define>BOOST_NOWIDE_DYN_LINK=1`) doesn't have any effect. What is the proper way to do this? Thanks, Alex