if using OpenGL, you could just fetch mouse-input, and draw buttons
using GL and capture input with these (like, checking if the mouse is
inside the button and the user is clicking, ...).
"proper" GUI turns into an ugly mess in a cross-platform setting (since
typically each OS does its own GUI stuff, which typically interact
poorly with GL).
but, if most of the user-input is done directly via SDL + OpenGL or
similar, then it is easier to be more portable (and the UI code doesn't
need to be OS-specific).
other major options include:
Java;
Adobe Flash;
JavaScript + WebGL;
...
but, personally, I would just assume stay in C or C++ land (these other
options make the developer pay in other ways...).