#819 Make update.bat more reliable

[Miguel Gimenez]

The patch in ticket 816 used the external command find.exe to detect the presence of 64 in the argument. Today user gd_on reported in the forum a problem with this, due to the presence of a Unix-like find.exe in his path prior to Windows' one. As this may occur with other users I have changed the detection method, see attaxched patch.