Log Message:
Do previous in a different way.  We want USE_GNU_ICONV to always use
a GNU libiconv, whether it be builtin or not, and PREFER_{NATIVE,PKGSRC}
can be used to decide which iconv we use if USE_GNU_ICONV isn't defined.
On NetBSD, the native iconv implementation (if it exists) is considered
GNUish enough unless USE_GNU_ICONV is explicitly defined.