MSVC warning C4388 in config.hpp

[pwiecz]

When T in e.g. an int, the comparison here: https://github.com/chriskohlhoff/asio/blob/master/asio/include/asio/impl/config.hpp#L40 compares signed value with an unsigned one, which makes MSVC to raise warning 4388

When executing the path may never be taken because of if (is_unsigned<T>::value), but the if clause is not constexpr so the compiler has to generate code for the branch and raise warning