Many of the functions we use implicitly are not part of the C standard
library so we have to specific that we're pulling from POSIX, X/Open,
BSD, or whatever standard we're using before we can rely on the
definition being available.
There's plenty of room for refactoring here since I mostly stopped at
just getting the functions being called to show up from their includes,
although I did replace rindex with strrchr since the latter is
standardized, and replaced `uint` with `unsigned` rather than trying to figure out
what header includes those.
(Updated to choose the right repository, this isn't libksysguard but instead kde/workspace/ksysguard)