[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:
            Type:  task              |                  Status:  closed
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Build System      |                 Version:  7.11
      Resolution:  fixed             |                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 Phyx-):

 I was wondering if the default flag for ./validate should be changed as
 well.
 Currently you can't do:
 {{{
 git clone --recursive git://git.haskell.org/ghc.git &&
 cd ghc
 ./validate
 }}}

 The workflow is a bit weird in which you need:

 {{{
 git clone --recursive git://git.haskell.org/ghc.git &&
 cd ghc
 ./boot
 ./configure --enable-tarballs-autodownload
 ./validate
 }}}

 Since `--enable-tarballs-autodownload` is ignored on anything not Windows
 and on Windows would only download the tarballs if missing, maybe it
 should be the default in `validate` ?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10705#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler