added support for kFreeBSD and GNU Hurd

[yangfl]

Ref: marcIhm/yabasic#29

[darealshinji]

I would add the _GNU_SOURCE like this:

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif

[darealshinji]

I think a correct check for kFreeBSD would be #if defined(__linux__) || ... || (defined(__FreeBSD_kernel__) && defined(__GLIBC__))

You need to check for __GLIBC__ to as FreeBSD also defines __FreeBSD_kernel__

[yangfl]

I think it's enough to check __GNU__ only since we do not really care whether it is __FreeBSD_kernel__ or something else, just GNU-like environment.