Give the RTS it's own configure script
Currently it doesn't do much anything, we are just trying to introduce it without breaking the build. Later, we will move functionality from the top-level configure script over to it. We need to bump Cabal for https://github.com/haskell/cabal/pull/8649; to facilitate and existing hack of skipping some configure checks for the RTS we now need to skip just *part* not *all* of the "post configure" hook, as running the configure script (which we definitely want to do) is also implemented as part of the "post configure" hook. But doing this requires exposing functionality that wasn't exposed before.