unix: Include fdatasync(2) on non-Apple systems

The function is defined in POSIX [0], but according to Gnulib docs [1],
it is missing or not declared on at least some versions of OS X and
FreeBSD. On Solaris, it is not a system call but is present as
fdatasync(3) [2].

[0] http://www.opengroup.org/onlinepubs/9699919799/functions/fdatasync.html
[1] https://www.gnu.org/software/gnulib/manual/html_node/fdatasync.html
[2] http://docs.oracle.com/cd/E36784_01/html/E36874/fdatasync-3c.html