Question: I don't ever recall coming across anything for C that's as extensive as C++'s Boost library.

Answer: 
I've never seen anything. Really, C++ templates are the enabling technology for STL and much of boost, and C doesn't have them. I've seen many C-interface libraries for things like image processing, 3D graphics, networking, etc., but never a real general-purpose library like boost. You can find pieces though - threading libraries, string libraries, etc. - that are similar to sections of boost.
Glib does compensate it partially in form of various unicode, string types. If you add Gobject you could get some object programming even some garbage collection.