Duncan Coutts authored Oct 29, 2016 and Herbert Valerio Riedel committed Dec 31, 2016

Going along with the existing approach of using a constraint rather than
altering the deps of custom Setup.hs scripts, but make the constraint
only apply to Cabal instances that are dependencies of Setup.hs scripts
not all instances of Cabal.

Also rearrange things a little with a dedicated solver policy function
for adding a min dep on Cabal versions for setup scripts.