BOOST_NO_CXX11_ALLOCATOR should be defined with libc++ in C++03 mode

BOOST_NO_CXX11_ALLOCATOR should imply no support for the C++11 allocator model. The C++11 allocator model requires a conforming std::allocator_traits which is only possible with C++11 template aliases since members rebind_alloc and rebind_traits require it.