Using __has_include to solve <filesystem>

// https://en.cppreference.com/w/cpp/feature_test
#ifdef __has_include
#  if __has_include(<filesystem>)
#    include <filesystem>  // gcc8 (Fedora29+)
     namespace fs = std::filesystem;
#  elif __has_include(<experimental/filesystem>)
#    include <experimental/filesystem> // gcc7 (Ubuntu 18.04)
     namespace fs = std::experimental::filesystem;
#  endif
#else
#error no __has_include
#endif