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

Answer: 
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.
or you can look at ccan <http://ccan.ozlabs.org/>

but no, there really isn't any complete package, glib comes closest