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>

configure

@@ -2461,7 +2461,13 @@ fi
 fdatasync=no
 cat > $TMPC << EOF
 #include <unistd.h>
-int main(void) { return fdatasync(0); }
+int main(void) {
+#if defined(_POSIX_SYNCHRONIZED_IO) && _POSIX_SYNCHRONIZED_IO > 0
+return fdatasync(0);
+#else
+#abort Not supported
+#endif
+}
 EOF
 if compile_prog "" "" ; then
     fdatasync=yes