the only icon that is installed correctly is xarchiver.png, the other files are not. i use path=/opt/xfce4, but they aren't installed at all. they aren't installed into a wrong directory.
as you already know, they are installed after all (sorry for the wrong information) but into a wrong directory, where gtk doesn't find them. all custom icons should be installed in $PREFIX/share/pixmaps/xarchiver/ to have one place to look for them and not 2 as it is at the moment while the application's icon goes to $PREFIX/share/icons/hicolor into the right size subdirectories. thunar for example does this too. but putting the custom icons to $PREFIX/share/xarchiver/pixmaps/ is not the right way.
Fixed in r23937. gt[xarchiver]$ ls /opt/xfce/bin/xarchiver /opt/xfce/bin/xarchiver gt[xarchiver]$ ls /opt/xfce/share/icons/hicolor/48x48/apps/xarchiver.png /opt/xfce/share/icons/hicolor/48x48/apps/xarchiver.png gt[xarchiver]$ ls /opt/xfce/share/pixmaps/xarchiver/ add.png add_button.png close.png extract.png extract_button.png gt[xarchiver]$ ls /opt/xfce/share/pixmaps/xarchiver/ add.png add_button.png close.png extract.png extract_button.png gt[xarchiver]$