>> Try adding #include <stdlib.h> directly above the
>> previous added #include
>> statement.
>
> Done. Building.
>
> Why are these includes needed to build on Slackware but not on Debian?
>

Good question.  The Debian compiler must be somewhat lax; Slackware seems
to be more strict.  The good thing is that if it compiles under Slackware,
then it will probably compile under most Linux systems in existence. 
Being strict is now always a bad thing... ;-)

Tim