Automatically run autoreconf -i when needed #3534

[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]

I've opened an upstream issue with Cabal about adding this support to the library directly:

haskell/cabal#4903

If this makes it into a future release of Cabal, we can conditionally disable this code based on the Cabal library version being used.

[mgsloan]

LGTM, good stuff!