GLib (not to be confused with glibc) from the GNOME project has a wide range of functionality -- generic lists, hash tables, strings etc.: https://en.m.wikipedia.org/wiki/GLib
Qt, I believe, also comes with a bunch of "standard library" stuff unrelated to UIs.
Yes, GLib basically implements its own full-fledged OOP system on top of C. At which point, you start asking yourself, why are you not using C++?
In my experience, most of the people who say they don't need OOP or generics end up implementing a limited (or even a full-fledged) version of these things themselves.
Qt, I believe, also comes with a bunch of "standard library" stuff unrelated to UIs.