Yes, its just for gtk, so you dont have to rebuild everything.
This is the gtk2.1 source and the patch. You could probobly just take the patch and apply it to a newer version of gtk, but im not positive.
1. Extract files.
2. copy the gtk2-2.2.1-menu-drop-shadow-hacky-hack.patch to: extracted dir/gtk/
3. apply the patch:
# patch gtkmenu.c gtk2-2.2.1-menu-drop-shadow-hacky-hack.patch
4. if patch succesfull then
# ./configure --prefix=/usr
# make install
5. restart Gnome or X