Tamar Christina authored Jul 30, 2015 and Ben Gamari committed Jul 30, 2015

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://ph...