RPM macros: Redefining %fedora now changes %dist
    
    To get a deterministic NEVR including the dist tag on a different release,
    it used to be enough to call `rpm` with:
    
        --define 'dist .fcXX'
    
    Later it got more complicated:
    
        --define 'dist %{?distprefix}.fcXX%{?with_bootstrap:~bootstrap}'
    
    And now it is even more complicated:
    
        --define 'dist %{!?distprefix0:%{?distprefix}}%{expand:%{lua:for i=0,9999 do print("%{?distprefix" .. i .."}") end}}.fcXX%{?with_bootstrap:~bootstrap}'
    
    It becomes more and more tedious. This change makes it possible to do this instead:
    
        --define 'fedora XX'
    
    Signed-off-by: There is nothing to sign