nasty hack to build when hS3 is not available

So, it would be nicer to just use Cabal and take advantage
of its conditional compilation support. But, Cabal seems to
lack good support for a package with an internal library that is used by
multiple executables. It wants to build everything twice or more.
That's too slow for me.

Anyway, fairly soon, I expect to upgrade hS3 to a requirment, and I
can just revert this.