Tk or Gtk2 is your easiest route. Your problem is how to design your app so that it remains responsive while your progressbar updates. There are a few commonly used methods, depending on how you will monitor the program to be run. If you can give more information, monks here can show you a way to do the callback. SDL is more for fast graphics, and would essentially involve reinventing the wheel, to make a progressbar using it.

So are you going to run your program through a piped open, IPC, thread, or what?

P.S. If you want an excellent intro to Gtk2, see novell-study-guide