[v3] use AM_ICONV

[Benjamin Kosnik]

> I have been able to reproduce such failures: the problem is at link 
> time, the _libiconv symbol remains undefined, apparently on such targets 
> libiconv should be linked to the final executable. Is that doable?

I think this should already be happening, due to LIBICONV substitution.

Maybe this isn't being picked up from the site.exp by dejagnu? Or 
something in conformance.exp needs to add in LIBICONV flags?

-benjamin