message: Use string_view if _HAS_CXX17 is true
  Current version of VC2017 is not setting __cplusplus to correct value, instead they use _MSC_VER, _MSVC_LANG and _HAS_CXX17 macros.
diff: // to detect a header that provides an implementation
  #if defined(__has_include)
  // Check for std::string_view (in c++17)
  - #if __has_include(<string_view>) && (__cplusplus >= 201606)
  + #if __has_include(<string_view>) && (__cplusplus >= 201606 || _HAS_CXX17)
  #include <string_view>
  namespace flatbuffers {
  typedef std::string_view string_view;
  