Remove GTK_LIB_DIR from appveyor it's no longer required
Remove GTK_LIB_DIR from appveyor it's no longer required

hmeyer/gdk,rust-gnome/gdk,rust-gnome/gdk,hmeyer/gdk,gkoz/gtk-rs-gdk,gkoz/rust-gnome-gdk,gkoz/gtk-rs-gdk,gkoz/rust-gnome-gdk