[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
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"9f7cdfee3e9f9ca6fbfa27d3b2dc2d86ac4ee226/ghc"
 9f7cdfee/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="9f7cdfee3e9f9ca6fbfa27d3b2dc2d86ac4ee226"
 Make configure error out on missing ghc-tarballs on Windows

 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.

 Test Plan:
 1. make sure ghc-tarballs folder is not present
 2. run ./configure which should fail giving an error that tarballs is
 missing and how to get it
 3. run ./configure --enable-tarballs-autodownload and the tarballs
 should be downloaded and configure finishes
 4. rerun the command in 3, no new download should be done.
 5. run configure without --enable-tarballs-autodownload, configure
 should finish correctly.

 Reviewers: bgamari, austin, thomie

 Reviewed By: thomie

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1108

 GHC Trac Issues: #10705
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10705#comment:2>
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