Move away from autoconf interpolation to generate `ghc.cabal`
configure is responsible for generating a number of
.cabal files for the build system:
However, this is a severe nuisance since whenever you update the
.cabal.in you need to rerun
configure, which usually invalidates the vast majority of your already-built tree. We really should move the generation of these files into the build system. This would be a good thing to do after we finally drop the Make build system.