This is a follow-up to fcc6b1de / D1608 which is made possible
by the recent Cabal update:

As `ghc-cabal` is called with `--with-gcc`, this gets passed to `./configure`
as `CC=...` argument. So we don't need to set `CC=...` ourselves explicitly again.

Prior to the changes in Cabal (pulled in via  0bf0cf93) and fcc6b1de,
`./configure` scripts would be called with a `--with-cc` argument followed by a
`--with-gcc` argument.

After this commit, `./configure` will be passed a single `CC=...` argument
constructed by the `Cabal` library.

Reviewed By: erikd

Differential Revision: https://phabricator.haskell.org/D1611