Turn 'configPrograms' field into a 'Last'-monoid

This implements the suggestions mentioned at
https://github.com/haskell/cabal/issues/3169#issuecomment-189281916

The main benefit of this change is turning 'ConfigFlags' into a uniform
product-type suitable for generic derivation of pointwise
`Semigroup`/`Monoid` instances.

NB: This changes the `Binary` serialisation of `ConfigFlags` since there's
now an additional `Maybe` inserted in `configPrograms`'s type