message: elle.log: add logs
diff: using namespace std::literals;
  +ELLE_LOG_COMPONENT("elle.log");
  +
  namespace elle
  {
  namespace log
  @@ -196,10 +198,11 @@ namespace elle
  std::unique_lock<std::recursive_mutex> ulock{log_mutex()};
  if (!_logger())
  {
  - auto level = os::getenv("ELLE_LOG_LEVEL", "LOG"s);
  + auto const level = os::getenv("ELLE_LOG_LEVEL", "LOG"s);
  _logger() = std::make_unique<TextLogger>(std::cerr, level);
  - _logger() = make_logger(os::getenv("ELLE_LOG_TARGETS",
  - "stderr://?" + level));
  + auto const ts = os::getenv("ELLE_LOG_TARGETS", "stderr://?" + level);
  + ELLE_DUMP("building main logger: {}", ts)
  + _logger() = make_logger(ts);
  }
  return *_logger();
  }
  @@ -222,13 +225,13 @@ namespace elle
  logger_add(std::unique_ptr<Logger> l)
  {
  std::unique_lock<std::recursive_mutex> ulock{log_mutex()};
  + ELLE_DUMP("add new logger: {}", l);
  auto log = dynamic_cast<CompositeLogger*>(_logger().get());
  if (!log)
  {
  // Create before, so that current logs apply.
  auto clog = std::make_unique<CompositeLogger>();
  - auto old = std::move(_logger());
  - clog->loggers().push_back(std::move(old));
  + clog->loggers().push_back(std::move(_logger()));
  log = clog.get();
  _logger() = std::move(clog);
  }
  