🔧 fix build in c++17 on GCC libstdc++ and on clang+libc++.

[daminetreg]

Dear @no1msd,

I've been working recently on a C++17 project and I've integrated your library. I also had to compile it under libc++ on macOS.

It happens that allowing copying of std::map with const keys is not a required feature from the C++ standard, libstdc++ has an implementation that allows it but libc++ not.

Therefore this patch does the following :

• allow mstch::map to be copied even on libc++
• fix ambiguous mstch::visit call to avoid recent changes in Boost.Variant (I used Boost 1.68.0).
• remove hardcoded compiler flags (actually these stuffs are better in CMake Toolchain File, letting code be tuned by compiler options without projects modifications).

If you don't like the third change I can push a toolchain file that sets the config or reinstate it with a cmake build option.

Thank you very much for the awesome mstch library. It is a really good foundation.
Cheers,