Clock Fix for GNAT

Ada.Calendar.Clock and Ada.Real_Time.Clock may leap forward by several
seconds.  This patch attempts to correct such behavior.  See
http://support.microsoft.com/directory/article.asp?ID=KB;EN-US;Q274323&

A simple test for the problem on a particular system is to time:
  End_Time := Ada.Calendar.Clock + 10.0;
  while Ada.Calendar.Clock < End_Time loop null;end loop;
and see whether it takes 10 seconds, or a fraction of a second, to run.