Fix issue with std::result_of being removed in c++20

[mikeseese]

This PR fixes #392 by adding a define to tell asio to use std::invoke_result instead of std::result_of; you can see the third party reference to these here: https://github.com/chriskohlhoff/asio/blob/master/asio/include/asio/detail/type_traits.hpp#L90-L96

IIRC I did not have issues compiling for Linux on 5.2.1, so I believe the new clang16 toolchain is defaulting to a c++ standard that removed std::result_of.

[getnamo]

excellent, thanks for catching this change