Ignore the -bindir option, meaningless on OpenBSD.

Ignore the -bindir option, meaningless on OpenBSD.

Lack of -bindir handling is not a problem right now because -bindir is
passed to cc(1) which ignores it, and because read(2) on /usr/local/bin
returns 0 right now, so ld(1) copes with it.  But there are plans to
make read(2) fail with EISDIR.

Input from and bulk builds thanks to ajacoutot@, ok ajacoutot@ espie@