deprecate fallback to ConfigureMake easyblock

@wpoely86, @stdweird: please review
True, but I doubt it's going to make a big difference if we hold EasyBuild v2.0 any longer...

You actually need to actively check for deprecation warnings, otherwise you'll miss them anyway.

The best way is to check with `--deprecated=2.0`, which turns deprecation warnings into errors, but this will only be useable in v1.16 (cfr. #1107)