[PATCH] Warn about builds with missing features

[Dirk Hohndel]

subsurface at henrik.synth.no writes:

> From: Henrik Brautaset Aronsen <subsurface at henrik.synth.no>
>
> I think it makes sense to warn when Subsurface has been built
> without all features enabled.  We could consider making
> these features mandatory instead.

I am more leaning towards making them mandatory (i.e., removing all the
ifdefs)

What do people think?

/D