`OPT_ARGUMENT(long, description)`::
Introduce a long-option argument that will be kept in `argv[]`.
+`OPT_NUMBER_CALLBACK(&var, description, func_ptr)`::
+ Recognize numerical options like -123 and feed the integer as
+ if it was an argument to the function given by `func_ptr`.
+ The result will be put into `var`. There can be only one such
+ option definition. It cannot be negated and it takes no
+ arguments. Short options that happen to be digits take
+ precedence over it.
+
The last element of the array must be `OPT_END()`.
The callback mechanism is as follows:
-* Inside `funct`, the only interesting member of the structure
+* Inside `func`, the only interesting member of the structure
given by `opt` is the void pointer `opt->value`.
`\*opt->value` will be the value that is saved into `var`, if you
use `OPT_CALLBACK()`.