message: minor cleanup num_threads
diff: @@ -9,8 +9,12 @@ class arg_num_threads : public int_argument {
  public:
  arg_num_threads() : int_argument() {
  _name = "num_threads";
  - _description = std::string("Number of threads available to the program.");
  + _description = std::string("Number of threads available to the program. For full effect, the model must be compiled with STAN_THREADS=true.");
  +#ifdef STAN_THREADS
  _validity = "num_threads > 0 || num_threads == -1";
  +#else
  + _validity = "num_threads == 1";
  +#endif
  _default = "1";
  _default_value = 1;
  _good_value = 1.0;
  