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.