With glibc, things like _POSIX_C_SOURCE and _ISOC99_SOURCE are
additive, but on FreeBSD they are mutually exclusive.  However, it
turns out we only need to define _POSIX_C_SOURCE and _XOPEN_SOURCE to
get all the C99 stuff we need too, so there's no need for any #ifdefs.

Submitted by: Gabor PALI <pgj@FreeBSD.org>