message: Update FLATBUFFERS_HAS_STRING_VIEW __cplusplus checks to the versions provided by the standard feature test 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 > 201402)
  + #if __has_include(<string_view>) && (__cplusplus >= 201606)
  #include <string_view>
  namespace flatbuffers {
  typedef std::string_view string_view;
  }
  #define FLATBUFFERS_HAS_STRING_VIEW 1
  // Check for std::experimental::string_view (in c++14, compiler-dependent)
  - #elif __has_include(<experimental/string_view>) && (__cplusplus > 201103)
  + #elif __has_include(<experimental/string_view>) && (__cplusplus >= 201411)
  #include <experimental/string_view>
  namespace flatbuffers {
  typedef std::experimental::string_view string_view;
  