message: Fix unused variable build warning on Mac.
diff: @@ -21,6 +21,7 @@ static const char *g_program_name = nullptr;
  static void Warn(const flatbuffers::FlatCompiler *flatc,
  const std::string &warn,
  bool show_exe_name) {
  + (void)flatc;
  if (show_exe_name) {
  printf("%s: ", g_program_name);
  }
  