message: elle.Duration: fix portability issue to Mingw
diff: @@ -172,7 +172,8 @@ namespace elle
  {
  // strlen("2017-07-31 05:24:53") = 20;
  auto buf = std::array<char, 24>{};
  - std::strftime(buf.data(), buf.size(), "%F %T", tm);
  + // Mingw does not support "%F %T".
  + std::strftime(buf.data(), buf.size(), "%Y-%m-%d %H:%M:%S", tm);
  os << buf.data();
  }
  if (this->time_microsec())
  