deprecate fallback to ConfigureMake easyblock

@wpoely86, @stdweird: please review
We can release EasyBuild v2.0 in which all the deprecated behaviour will trigger an error, but still allow to escape via `--deprecated=1.0` for example.

And in EasyBuild v2.1, we kick out the deprecated code and break that stuff forever...

That makes the transition more gradual?