e.g., `make distro/icewm' instead of `make distro/icewm.iso'
would be "successful" as there's a corresponding target indeed
but the "success" would just mean building the configuration
without running the actual build.

Thanks cas@ for hitting this issue and reporting it.

PS: note that the initial flagless implementation turned out
    to produce false positives for e.g. `make distclean'.