pre-710 compatibility stuff

[Herbert Valerio Riedel]

On 2015-10-24 at 12:19:38 +0200, lennart spitzner wrote:
> hi,
>
> should we use
> #if __GLASGOW_HASKELL__ < 710
> or
> #if !MIN_VERSION_base(4,8,0)
> ?
>
> currently both are used in different places, and i have no idea if
> there is a difference/if it matters. if it does not, i suggest
> conventionizing one of the two.

IMO, guarding with MIN_VERSION_base() ought to refer to properties of
the `base` library (e.g. availability of class instances or other
symbols), while guarding with __GLASGOW_HASKELL__ should refer to
properties (e.g. availability/semantics of language pragmas) and use of
a specific GHC version. While we currently have a 1:1 relationship
between base and GHC, this wasn't always the case (and it may make sense
to relax this constraint again at some point in the future).

Cheers,
  hvr