message: Fix building with GCC 8.x
diff: @@ -61,7 +61,9 @@ class transaction::impl {
  if (!rc) {
  return {};
  }
  - return {stdx::make_unique<read_concern::impl>(libmongoc::read_concern_copy(rc))};
  + class read_concern rci(
  + stdx::make_unique<read_concern::impl>(libmongoc::read_concern_copy(rc)));
  + return stdx::optional<class read_concern>(std::move(rci));
  }
  void write_concern(const class write_concern& wc) {
  @@ -74,7 +76,9 @@ class transaction::impl {
  if (!wc) {
  return {};
  }
  - return {stdx::make_unique<write_concern::impl>(libmongoc::write_concern_copy(wc))};
  + class write_concern wci(
  + stdx::make_unique<write_concern::impl>(libmongoc::write_concern_copy(wc)));
  + return stdx::optional<class write_concern>(std::move(wci));
  }
  void read_preference(const class read_preference& rp) {
  @@ -87,7 +91,9 @@ class transaction::impl {
  if (!rp) {
  return {};
  }
  - return {stdx::make_unique<read_preference::impl>(libmongoc::read_prefs_copy(rp))};
  + class read_preference rpi(
  + stdx::make_unique<read_preference::impl>(libmongoc::read_prefs_copy(rp)));
  + return stdx::optional<class read_preference>(std::move(rpi));
  }
  mongoc_transaction_opt_t* get_transaction_opt_t() const noexcept {
  