Looking at the mailing list archives, this has been a
problem on and off for a long time. I first tried
compiling with --disable-iconv, which fixed the error,
but then some of the plugins complained that they
couldn't convert strings. So I decided against that.
I've managed to work around it by as follows:
cp mac%alt.so MACINTOSH%UTF-8.so
cp alt%mac.so UTF-8%MACINTOSH.so
This seems to shut up all the messages. But I'm
nervous, since I suspect that "alt" probably isn't
the whole of UTF8...