[boost] [config] Should BOOST_NO_CXX11_ALLOCATOR be undefined for libcpp without std=c++11

I observed the absence of BOOST_NO_CXX11_ALLOCATOR with clang and libc++ without -std=c++11. Is this intended? Right now BOOST_NO_CXX11_ALLOCATOR's absence is (in my view) an indication of whether C++11 allocators, including class template std::allocator_traits, are supported - and without -std=c++11 neither could be. Glen