ITEM_WORN had been removed along with a few other things that appeared to no longer be used by the Smaug code. build.c in the stock packages still shows that to coorespond to the _worn slot, which was their way of saying it's not used. The slot can be put back if need be, and in thinking about it, ITEM_CLOTHING makes perfect sense, so it will probably make its way back into the next release using that name.
So to put it back in the meantime, you'd just do what you've already described. Change it in olc.h and build.c and it should work from there.