The filename will be prefixed by passing the filename along with
the prefix argument of `parse_options()` to `prefix_filename()`.
-`OPT_ARGUMENT(long, description)`::
+`OPT_ARGUMENT(long, &int_var, description)`::
Introduce a long-option argument that will be kept in `argv[]`.
+ If this option was seen, `int_var` will be set to one (except
+ if a `NULL` pointer was passed).
`OPT_NUMBER_CALLBACK(&var, description, func_ptr)`::
Recognize numerical options like -123 and feed the integer as