configure: stop enabling fdatasync on OSX

configure: stop enabling fdatasync on OSX

Change configure compile probes to fail on implicit declarations. This
(correctly) stops us from enabling fdatasync on OSX which was
problematic because we were actually calling a syscall stub with
arguments different to what was declared in its prototype (see
https://github.com/gbrault/picoc/issues/145#issuecomment-89734655 and
https://gitlab.freedesktop.org/xdg/shared-mime-info/issues/7 ).

Fixes: https://github.com/axboe/fio/issues/834 ("Drop macOS support for
fdatasync")

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>