I can compile FLTK programs now. Here is the Hello World program. I have no idea how it works but if I spend months learning about it then maybe I could do similar things with it as I did SDL and Allegro. It’s probably overpowered for what I need since I mostly just care about drawing shapes to the window.
The reason I decided to try FLTK was because of Tiny Core Linux using it for its window manager. It’s smaller than other toolkits so theoretically it would be easier to learn.