Hate to revive my own post, but I've been patiently waiting for about half a year. It's next to impossible to find a C only engine. Here's my results(feel free to add any others pleas!) to start off the C only engine database.
Horde3D(is C++, but has .dll to be called from C)
Blitz3D SDK(also a .dll that can be called from C)
I'm hoping since the database is currently being updated, this can get some attention. Maybe just add the option for "C only" and that way nothing will break. Then slowly over time, users like myself can edit the quake entires, etc. so they are changed to "C only" or "Useable in C" or some type of equivilent entry.