Build: automatically generate list of files to be translated in configure
    
    configure is easiest option how to automatically generate POTFILES.in.
    Attempts to add it to po/Makefile* have big potential to create cyclic
    depedencies and cause other trouble.
    
    Given how rare operation adding a source file is, I think it is sufficient
    to document that configure needs to be run again after adding a source file
    with translatable strings.
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>