message: Fix build with gcc version 7.4.0
diff: @@ -199,7 +199,7 @@ namespace flatbuffers {
  // 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 || _HAS_CXX17)
  + #if __has_include(<string_view>) && (__cplusplus >= 201606 || (defined(_HAS_CXX17) && _HAS_CXX17))
  #include <string_view>
  namespace flatbuffers {
  typedef std::string_view string_view;
  