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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

See merge commit 66693401

This commit also adds a check for a left-over testsuite/.git folder to sync-all

This way, the first time sync-all is called after updating to a
post-testsuite-merge (see #8545) state of ghc.git, the sync-all script
aborts with an error message if a `testsuite/.git` folder is detected and
thus forces the user to take action.
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

(this refs #8121 for testing purposes)

Signed-off-by: Austin Seipp <aseipp@pobox.com>