configure: Fix check for fdatasync()

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: Alexandre Raymond <cerbere@gmail.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
