Check to see if we are running under MS-Windows.
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 Syntax

        GT_IsWin() --> lIsWin

 Arguments:

      None.

 Returns:

      A logical value, .T. if we are running under MS-Windows, .F. if
      not.

 Description:

      GT_IsWin() can be used to check if your application is been run
      under MS-Windows. This could come in handy if you want to provide
      your users with extra features such as pasting text into the
      Windows clipboard.

 Examples:

      // Check if we are running under MS-Windows.

      If GT_IsWin()
         ? "Oh well, never mind....."
      Else
         ? "Ahhh! This ain't Windows.... ;-)"
      EndIf

 Source: ISWIN.C

 Author:  Dave Pearson