message: Support older compiler/library use of std::string/string_view.
  Older libraries don't support direct conversion from absl::string_view to
  std::string.  Use 2-iterator range construction instead.
  fixes
diff: @@ -52,7 +52,7 @@ const SubcommandEntry& SubcommandRegistry::GetSubcommandEntry(
  const auto iter = commands.find(command); // heterogenous lookup
  #else
  // without heterogenous lookup
  - const auto iter = commands.find(std::string(command));
  + const auto iter = commands.find(std::string(command.begin(), command.end()));
  #endif
  if (iter == commands.end()) {
  // Command not found, print help and exit non-zero.
  @@ -67,7 +67,7 @@ absl::Status SubcommandRegistry::RegisterCommand(
  #if __cplusplus >= 201402L
  name, // heterogenous lookup
  #else
  - std::string(name), // without heterogenous lookup
  + std::string(name.begin(), name.end()), // without heterogenous lookup
  #endif
  command);
  if (!p.second) {
  