Add MZ_TRY_EXTFLONUMS to OpenBSD.
Add MZ_TRY_EXTFLONUMS to OpenBSD.

Also add an error to unsupported platforms (copied from FreeBSD).

Merge to 5.3.4

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket