What sort of options are there these days for this sort of thing? I'm looking for a C-compatible API (or D, since that's technically the language I'm using), and the field seems a little sparse. Everyone seems to have fallen in love with IMGUI-style libraries. I can see why; mocking things up with one (imgui in particular, with a somewhat-clunky C binding) was a breeze. However, the presentability of the result leaves a bit to be desired. My game is fairly UI-driven, and I'm looking for something I can skin and animate for that purpose, not so much in the way of tools development.
The place I last worked at used an in-house solution, and it seems like many commercial games use scaleform. Almost everything else I've seen is written in C++, like CEGUI and LibRocket. Is there anything out there that supports C? Should I just build my own? I've heard that's an extremely laborious task.