HTCONDOR-776: MacOS lacks fdatasync.
HTCONDOR-776: MacOS lacks fdatasync.

Surprisingly, `fdatasync` is a syscall in the kernel but not
defined in system headers.

htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor