Ensure that fdatasync is properly declared as on Mac OS X, the
function is available but there is no prototype. Also, port a
fix for a warning from the InnoDB plugin over to the builtin. 

configure.in:
  Check that fdatasync is declared.
mysys/my_sync.c:
  Use fdatasync only if it is declared.
storage/innobase/include/ut0dbg.h:
  Port over from the plugin a fix for a warning.