SDL-widgets
SDL-widgets is a GUI toolkit on top of the SDL libraries. The toolkit is written in C++. It's not especially intended for games, it aims at normal applications with a possibly complicated SDL_Surface hierarchy, where surfaces can be hidden or shown, with sliders, menus, text areas, etc. Threads are supported, they communicate with the main thread via messages.
This is version 2, using C++11 features. Especially the lambda closures are very handy for building a GUI. This implicates that it is incompatible with last version 1, although porting is straightforward. If the compiler doesn't complain, then your program will run as expected.