CXX-1688 Fix building with GCC 8.x

Author: Piotr Szczepanski

src/mongocxx/options/private/transaction.hh

@@ -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 {