The iconv(3) function on NetBSD has a custom prototype due to old mistake in
the POSIX specification. The issue has been resolved but we keep using original
form with a constified parameter.
For now, there's only single OS-dependent bit of code, it it should be
handled on FreeBSD just like on Linux. While here, change macro testing
from #if XXX to #if defined(XXX), this is clener and not prone to
"undefined macro" errors
