[v3] use AM_ICONV

[Benjamin Kosnik]

> Thanks, I can probably further investigate the issue along that 
> direction relatively soon (conditional to access to darwin-powerpc 
> hardware ;) Maybe Kaveh can do the same on Solaris?!?

Thanks Paolo. My access to darwin has been dead since my brother's 
machine in australia went down.

To be clear, it looks like this patch has two issues remaining:

1) --with-iconv-prefix and negations not working

2) testsuite issues with darwin/solaris not linking/not linking correct 
libiconv

-benjamin