Some may remember me from a few years back, but I put together a UI library & tool specifically aimed at game developers. It's purpose was (and still is) to be cross-platform and engine-agnostic, by being developed in as vanilla C/C++ as possible, while offloading the engine-specifics to the developer. All the actual rendering, sound, etc is handled by the developer - OtterUI just tells you what to render and when. Same goes for file I/O, audio, and so on.
To show you what OtterUI can do, check out Skullgirls (http://skullgirls.com/)- its entire interface was built using the library & tool.
Fast forward a few years - after trying to build a company out of it, and not going very far, I've decided to make it Open Source and let the community benefit overall. OtterUI source & latest distributables can be found on github at: https://github.com/Grafalgar/OtterUI, and is distributed under the MIT License.
Let me know if anyone has any questions, otherwise, enjoy.