versionCheckHook: emit warning when falling back to pname

    elif [[ ! -z "${NIX_MAIN_PROGRAM-}" ]]; then

        cmdProgram="${!outputBin}/bin/${NIX_MAIN_PROGRAM}"

    elif [[ ! -z "${pname-}" ]]; then

        echo "versionCheckHook: Package \`${pname}\` does not have the \`meta.mainProgram\` attribute." \

            "We'll assume that the main program has the same name for now, but this behavior is deprecated," \

            "because it leads to surprising errors when the assumption does not hold." \

            "If the package has a main program, please set \`meta.mainProgram\` in its definition to make this warning go away." \

            "Should the binary that outputs the intended version differ from \`meta.mainProgram\`, consider setting \`versionCheckProgram\` instead." >&2

        cmdProgram="${!outputBin}/bin/${pname}"

    else

        echo "versionCheckHook: \$NIX_MAIN_PROGRAM, \$versionCheckProgram and \$pname are all empty, so" \