Adjust make knobs to know that WITH_ICONV installs a /usr/bin/iconv

Remove it if the knob isn't tweaked.  Certain ports seem to think that
if /usr/bin/iconv exists, then libc has built in libiconv things and will
blow up pretty nicely when built.

Reviewed by:	gjb@
MFC after:	2 weeks
MFV r254011:

This change have no effect to FreeBSD but integrated for
completeness.

Illumos ZFS issues:
  348 ZFS should handle DKIOCGMEDIAINFOEXT failure