[GHC] #10705: make configure warn when ghc-tarballs is missing on windows and provide guidance to download it.

[GHC]

#10705: make configure warn when ghc-tarballs is missing on windows and provide
guidance to download it.
-------------------------------------+-------------------------------------
        Reporter:  Phyx-             |                   Owner:  Phyx-
            Type:  task              |                  Status:  patch
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Build System      |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Windows           |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #9218             |  Differential Revisions:  Phab:D1108
-------------------------------------+-------------------------------------
Changes (by Phyx-):

 * status:  new => patch
 * differential:   => Phab:D1108


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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10705#comment:1>
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