Substituting std::holds_alternative

[afalkenhahn]

I'm backporting some C++17 code to C++14 and I'm using mapbox::variant as a replacement for std::variant which is unavailable in C++14. But now I've stumbled across some code that uses std::holds_alternative. AFAICS, this is currently unavailable in mapbox variant, isn't it? How could I do what std::holds_alternative does with mapbox variant?