[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:  new
              Priority:  normal      |         Milestone:  7.12.1
             Component:  Build       |           Version:  7.11
  System                             |
              Keywords:              |  Operating System:  Windows
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:  #9218
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Currently checking out the source on windows requires two git checkouts.
 One for the GHC sources and one for the GHC-tarballs.

 This patch will make configure issue an error if compiling under windows
 and the GHC-tarballs folder is missing.

 On failure the user is told which command they need to run to get the
 tarballs or if they want configure to handle it for them configure provide
 the `--enable-tarballs-autodownload` flag.

 This is one part of the bigger #9218 patch which is being flit up to get
 it through.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10705>
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