Removed _INCOMPAT_ICONV.

The problem is Darwin's libiconv does not have symbols for libiconv_<name>
(e.g. libiconv_open), but iconv_<name> (e.g. iconv_open).
BUT when there's pkgsrc/converters/libiconv installed instead, it doesn't
have symbols for iconv_<name>, but libiconv_<name>.
Some packages auto-configure looks for libiconv_open (like glib2), others
look for iconv_open (like proftpd), and there's a conflict.
The solution is to replace libiconv_open with iconv_open with SUBST framework.

(adam)