[boost] libcpp.hpp always defines BOOST_NO_CXX11_HDR_ATOMIC

The libc++ config header, libcpp.hpp, always defines BOOST_NO_CXX11_HDR_ATOMIC, but the libc++ that - I presume - ships with "Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)" appears to have a functional <atomic>.