Update benchmark

Makefile

@@ -1,6 +1,8 @@
+CXX=g++
+CXXFLAGS=-Wall -Wextra -O3
 
 benchmark: benchmark.cpp
-	g++ -Wall -Wextra -o benchmark benchmark.cpp
+	$(CXX) $(CXXFLAGS) -o benchmark benchmark.cpp
 
 clean:
 	rm -rf benchmark