Use the Builder from bytestring >= 0.10

`bytestring` >= 0.10 provides its own `Builder`. This patch makes `aeson` use that `Builder` but if users don't have `bytestring` >= 0.10 they can configure the package with `-fblaze-builder` which enables `aeson` to be build with `blaze-builder` and older versions of `bytestring`.

The motivation for this patch is that this makes it easier to propose `aeson` for the Haskell Platform because `blaze-builder` is not in the HP.