I have also run into this problem. I have tried at different moments in time Gtk, it's C++ bindings, wxWidgets, Java+AWT/Swing, Qt.
I couldn't agree more that all of them have major drawbacks. We need a good, simple to use, well-designed, fast cross-platform C library.
Do you care to start doing it? Other than doing your own, I think the most sensible choice is wxWidgets, which, from my point of view, only has one big problem: the API (i.e. code doesn't look pretty, bad C++ (no exceptions), and it can take a while to compile).