The idea here is to build cabal-helper with whatever version of Cabal
the user happens to be using (which we find by looking at
dist/setup-config) at runtime.
This way we can support literally any version of Cabal as long as the
actual cabal-helper still compiles.
I tried to only use interfaces in Cabal that have been there since at
least 1.16 so I'm hoping this shouldn't break too much.
