message: Work around build error on gcc < 7.4.
diff: @@ -79,7 +79,7 @@ static absl::node_hash_map<string, std::atomic<bool>>* fuel_ever_consumed;
  static absl::node_hash_map<string, std::atomic<int64>>* global_fuel;
  // If we're using thread-local fuel, this stores it.
  -static thread_local absl::optional<
  +static thread_local std::unique_ptr<
  absl::node_hash_map<string, std::atomic<int64>>>
  thread_fuel; // NOLINT (global variable with nontrivial destructor)
  @@ -519,7 +519,7 @@ xla::DebugOptions GetDebugOptionsFromFlags() {
  void ResetThreadLocalFuel() {
  std::call_once(flags_init, &AllocateFlags);
  - thread_fuel.emplace();
  + thread_fuel.reset(new absl::node_hash_map<string, std::atomic<int64>>());
  CHECK(initial_fuel != nullptr);
  for (const auto& kv : *initial_fuel) {
  thread_fuel->emplace(kv.first, kv.second);
  @@ -531,7 +531,7 @@ bool ConsumeFuel(absl::string_view pass, bool* just_ran_out) {
  if (just_ran_out != nullptr) {
  *just_ran_out = false;
  }
  - auto* fuel_pool = thread_fuel.has_value() ? &*thread_fuel : global_fuel;
  + auto* fuel_pool = thread_fuel ? thread_fuel.get() : global_fuel;
  if (fuel_pool->empty()) {
  return true;
  }
  