wbthrottle: use feature check for fdatasync

Checking for fdatasync uses the same approach as the qemu configure
script. The relevant commit is d1722a27f552a22561104210e0afad4577878e53.
Here is a copy of the commit message which explains the check:

Under Darwin, a symbol exists for the fdatasync() function, so that our
link test succeeds. However _POSIX_SYNCHRONIZED_IO is set to '-1'.

According to POSIX:2008, a value of -1 means the feature is not
supported.
A value of 0 means supported at compilation time, and a value greater 0
means supported at both compilation and run time.

Enable fdatasync() only if _POSIX_SYNCHRONIZED_IO is '>0'.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #919 from ceph/port/fdatasync

wbthrottle: use feature check for fdatasync

Reviewed-by: Sage Weil <sage@inktank.com>