[v3] use AM_ICONV

[Paolo Carlini]

Benjamin Kosnik wrote:

> There's nothing to revert to: it's been broken forever, although only 
> obviously so on non-GNU platforms like solaris.

In other terms the old GLIBCXX_CHECK_ICONV_SUPPORT didn't work? It had 
the nice comment:
   
    # From Bruno Haible's AM_ICONV, but without link tests.

Anyway, if things were already broken, I don't understand the recent 
message from Ian?!?

Paolo.