How should mock behave when DNF is not available?

[Miroslav Suchy]

I am working on switching rawhide targets to DNF. However on EL* is DNF 
not available.

Therefore you will not be able to build rawhide packages on EL7- using 
standard mock configuration.

I see several options to offer to user:

1) Discourage user from doing that by providing no hints how to 
workaround it.

2) Tell him that DNF is not available and he must run mock with "--yum" 
option. And warn him that he may get different depsolving.

3) Tell him that DNF is not available and fall back automatically to yum 
and warn him that he may get different depsolving.

Unless somebody will have better idea I would go with (2).

Ideas? Comments?

Mirek