Fix non-buildable bug in Cabal <= 1.22

[snoyberg]

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[snoyberg]

@borsboom If we're keeping a maintenance branch for future 1.6 point releases, this is a could candidate for inclusion, as it's a relatively low impact change.

[mgsloan]

Hmm, I don't see the reasoning of using a GHC version as a proxy for the cabal version. AFAIU you can use Cabal 2 with GHC 7.8

[snoyberg]

I considered going that route, but it's a much bigger diff, and gets into a bit of a circular issue, since we need to know the version of the Cabal library before we've finished calculating the build plan. The way I've written it, we'll occasionally (but rarely) get some unnecessary extra packages built if someone is using old GHC + new Cabal, but won't get any breakage (since you can't use old Cabal with a new GHC).

[mgsloan]

I see! Alright fair 'nough

[borsboom]

If we're keeping a maintenance branch for future 1.6 point releases, this is a could candidate for inclusion, as it's a relatively low impact change.

The stable branch is perfect for this. Feel free to retarget this PR to that branch, or cherry-pick it after merging to master.

[snoyberg]

Rebased against stable. After Travis passes, I'll merge and then merge stable to master.

[snoyberg]

For posterity, here's the original Cabal bug: haskell/cabal#1725