Ryan Scott authored Feb 14, 2019 and Marge Bot committed Feb 20, 2019

Along the way, I discovered that `template-haskell.cabal` was
hard-coding the GHC version (in the form of its `ghc-boot-th` version
bounds), so I decided to make life a little simpler in the future by
generating `template-haskell.cabal` with autoconf.