keep in mind on your distro, packages ( typically libraries) are split. for example, for gtk-2.4 will have two packages ( the names will differ from my example slightly):
gtk2-2.4.0-i386 - this it the package that contains the libraries needed to use gtk.
gtk2-2.4.0-devel-i386 - this package contains the files needed to build applications that use gtk2. These files include documentation, include files ( headers), and the pkg-config files that pkg-config uses ( not all libraries use pkg-config, but pretty much all of the gnome-2.x ones do).