configury: add missing fdatasync declaration if necessary.

Fixes #172.
* configure.ac (AC_CHECK_DECLS_ONCE): Check for missing fdatasync
declaration, e.g. on Mac OS 10.9.
* ext/posix/unistd.c (Pfdatasync): Roll our own to save a compiler
warning if the system didn't supply one.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>