[v3] use AM_ICONV

[Benjamin Kosnik]

> I haven't really been following the iconv discussion.  What is the
> current status?  I get a failure when building a cross-compiler:
> 
> checking for iconv... configure: error: Link tests are not allowed after GCC_NO_EXECUTABLES.
> make: *** [configure-target-libstdc++-v3] Error 1

Although I can see why you might get this, I am unable to reproduce this 
with today's mainline hosted on x86-linux targeting arm-elf (which is 
testing fine). I'm not super keen on checking in fixes to this without 
being able to reproduce your issue.

Please specify your host and configure line.

best,
benjamin