(XS) RFR: 8077674: BSD build failures due to undefined macros

[David Holmes]

webrev:

http://cr.openjdk.java.net/~dholmes/8077674/webrev/

 From the original error report:
> "-Wundef option causes GCC to warn whenever it encounters an identifier
> which is not a macro in an ‘#if’. "
>
> Hence we should be using
>
> #elif defined(__FreeBSD__)
>
> etc in this context.

I also founds similar suspect usages in os_linux.cpp and os_windows.cpp.

Thanks,
David