[FindIconv] expand -t4
[FindIconv] fix it for some of darwinds (that have installed iconv from ports)

If you install libiconv from ports, than you will have next iconv
headers:
/usr/include/iconv.h
/opt/local/include/iconv.h # for libiconv

And it seems that the second one is used by gcc, and for this version we
need LIBICONV_PLUG.

Anyway adding LIBICONV_PLUG mustn't break anything for any other usual
configuration