Replace cpp_version.h with a config file

[alandefreitas]

The problem

The cpp_version.h file for custom feature detection is much more than we really need. We don't use most macros it provides and many macros are falling back to the C++ standard version instead of making more elaborate tests. A simple config file would work better for our needs.

Proposed solution

Create a detail/config.hpp with the macros we might need. Remove cpp_version.h and hedley.h from our implementation details.

Alternatives I've considered

A version of cpp_version.h.

Additional context

Implementing a version of cpp_version.h as a separate library still has a lot of merit and would be quite useful. In this separate library, we could ensure the traits work for all compilers we need and then reuse that as needed.