configure on Windows now downloads a mingw distribution on the fly if needed.

Summary:
Also, migrated away from the rubenvb mingw distribution to a more up-to-date
and semi-official mingw-builds. Updated the version of gcc to 4.8.3.

This change obviates the need for a ghc-tarballs repository.

Also removed use of an embedded perl (also from ghc-tarballs) since it is provided by msys2.

Reviewers: austin

Reviewed By: austin

Subscribers: awson, hvr, thomie, carter, simonmar

Differential Revision: https://phabricator.haskell.org/D339

GHC Trac Issues: #9218