Fix build with gcc-7 on linux (WIP)

[falkTX]

Not very clean, needs improvement.
But already gives you an idea of what is needed for compatibility

[paulfd]

Hi, thanks. I like the latest version with the __has_include.

If you want to make it completely compatible with gcc-7 there will be some issues with <optional> and <string_view>. That being said, since I already rely on abseil, they provide c++11 versions of both and they default to the standard library when compiled with c++17. Maybe using absl::optional and absl::string_view everywhere would be easier for older compiler versions.