GLib is a portability/utility library for C which forms the basis of the GTK+ graphical toolkit. It can be used as a standalone library.
It contains portable wrappers for managing directories. See Glib File Utilities documentation for details.
Personally, I wouldn't even consider writing large amounts of C-code without something like GLib behind me. Portability is one thing, but it's also nice to get data structures, thread helpers, events, mainloops etc. for free
Jikes, I'm almost starting to sound like a sales guy :) (don't worry, glib is open source (LGPL) and I'm not affiliated with it in any way)