ASIO_HAS_STD_ADDRESSOF not set for Visual Studio

[Nairou]

Both VC10 and VC11 support std::addressof(). However, this is not detected by ASIO in addressof.hpp, resulting in the Boost file "boost/utility/addressof.hpp" being included, despite ASIO_STANDALONE being defined.

[chriskohlhoff]

Fixed in 308c775.