This ensures that the check_function_exists() for fdatasync in the
libeio CMakeLists.txt is not run, since that fails on some versions of
OSX (http://public.kitware.com/Bug/view.php?id=10044).
Squashed commit of ca128f7dcd28cbcfba154c8577ed54d4aa71dd02 with
contributions from Mark Constable (markc@renta.net) and Daniel Gröber
(darklord@darkboxed.org).