[BUILD] ensure that users don't build without setting the target anymore.

[BUILD] ensure that users don't build without setting the target anymore.

Too often, people report performance issues on Linux 2.6 because they don't
use the available optimizations. We need to ensure that people are aware of
the available features, and for this, we must force them to choose a target
OS (or "generic"), but at least prevent them from blindly building for a
generic target.