Asio's implementation can use std::atomic<> in preference to boost::detail::atomic_count.

Support for the standard atomic integer template is automatically enabled for g++ 4.5 and later, when the -std=c++0x or -std=gnu++0x compiler options are used. It may be disabled by defining ASIO_DISABLE_STD_ATOMIC, or explicitly enabled for other compilers by defining ASIO_HAS_STD_ATOMIC.