`int_var` is set to `integer` with `--option`, and
reset to zero with `--no-option`.
-`OPT_SET_PTR(short, long, &ptr_var, description, ptr)`::
- Introduce a boolean option.
- If used, set `ptr_var` to `ptr`.
-
`OPT_STRING(short, long, &str_var, arg_str, description)`::
Introduce an option with string argument.
The string argument is put into `str_var`.