build: disable iconv by default.

It seems to break a lot on MacOS because of a mixup between multiple
iconv libraries. The issue is that the configure check link with no
particular library path (so it uses /usr/lib/libiconv.dylib where all
the symbols are defined). OTOH, the final build might link with extra
library paths, such as the ports lib directory (typically you get
-L/opt/local/lib because of an extra library such as SDL). Whatever this
option position (before or after -liconv), it will try to link with
/opt/local/lib/libiconv.dylib, and the issue is that this version has
the same symbols as the ones in /usr/lib/libiconv.dylib except that they
all start with 'lib' (libiconv_open, libiconv_close, ...). I don't plan
to try to workaround this mess, so any better solution is welcome.
