software/powerdns: declare a minium gcc version

This software can use system gcc. On test nodes we observed successful
builds when system gcc is 8.3.0 and failures with system gcc is 6.3.0:

    checking whether g++ supports C++17 features with -std=c++17... no
    checking whether g++ supports C++17 features with +std=c++17... no
    checking whether g++ supports C++17 features with -h std=c++17... no
    checking whether g++ supports C++17 features with -std=c++1z... no
    checking whether g++ supports C++17 features with +std=c++1z... no
    checking whether g++ supports C++17 features with -h std=c++1z... no
    configure: error: *** A compiler with support for C++17 language features is required.
    ...
    While:
      Installing powerdns.