Using the gtk+-1.2.10-*-5 and glib-1.2.10-*-3 packages from Slackware 14.2 produces an error:

| multiple definition of `g_bit_storage'

gdk_imlib isn't needed for kuickshow, and imlib can be built without it by moving gtk-config out of the PATH. Or removing those packages or upgrading to gtk+-1.2.10-*-7 and glib-1.2.10-*-7 from Slackware-current produces a successful build.

So if you really want to build gdk_imlib, try using those Sl-current packages, but be aware of this note from the Imlib README:

"Also note that gdk-imlib WILL NOT WORK with GTK2. It won't even compile.
 This is not a bug. This is a feature. If you need an image loader for GTK2, use gdk-pixbuf."