SetupWrapper: Assume that old Cabals bootstrap.

[23Skidoo]

See e2bf243#commitcomment-24487898.

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
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[mention-bot]

@23Skidoo, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dcoutts, @ezyang and @arianvp to be potential reviewers.

[23Skidoo]

Blocked by this strange GHC 8.0.2 failure:

stderr:
--- /home/travis/build/haskell/cabal/cabal-testsuite/PackageTests/NewBuild/T4375/cabal.dist/cabal.out.normalized	2017-09-22 16:41:40.699667452 +0000
+++ /home/travis/build/haskell/cabal/cabal-testsuite/PackageTests/NewBuild/T4375/cabal.dist/cabal.comp.out.normalized	2017-09-22 16:41:40.699667452 +0000
@@ -7,6 +7,7 @@
  - old-locale-1.0.0.7 (lib) (requires download & build)
  - old-time-1.1.0.3 (lib) (requires download & build)
  - a-0.1 (lib:a) (first run)
+writing <ROOT>/cabal.dist/home/.cabal/packages/test-local-repo/old-time/1.1.0.3/old-time-1.1.0.3.tar.gz
 Configuring library for old-locale-1.0.0.7..
 Preprocessing library for old-locale-1.0.0.7..
 Building library for old-locale-1.0.0.7..
*** unexpected failure for PackageTests/NewBuild/T4375/cabal.test.hs

[23Skidoo]

OK, that test runs only when GHC == 8.0.*, which explains why it fails only on that configuration...

[23Skidoo]

writing <ROOT>/cabal.dist/home/.cabal/packages/test-local-repo/old-time/1.1.0.3/old-time-1.1.0.3.tar.gz

What's weird is that that message comes from a call to info, which shouldn't produce marked output...

[23Skidoo]

Failure seems to be intermittent... I think there's a race condition that could be worked around by setting stdout to block buffering and flushing after each hPutStr... or maybe we should just run that test with -j1.

[23Skidoo]

Test failure is intermittent and unrelated to the change, see #4376.

[23Skidoo]

Also cherry-picked into 2.0.