backend configure update

[Adam Fedor]

Just to prove I can't do things half-way, I found a little bug in 
back/configure, and I ended up rewriting the whole thing. So if a few 
people could test that it still works as well as before, that would be 
good.

 Also, I fixed the art backend to 'fall back' to xlib if all the 
dependancies are not found. And added a default nfont. So now it should 
be trivial to make the art backend the default one when we can agree on 
that.