switch off assertions for benchmarks

Makefile

@@ -87,7 +87,7 @@ pretty:
 # benchmarks
 json_benchmarks: benchmarks/benchmarks.cpp benchmarks/benchpress.hpp benchmarks/cxxopts.hpp src/json.hpp
 	cd benchmarks/files/numbers ; python generate.py
-	$(CXX) -std=c++11 $(CXXFLAGS) -O3 -flto -I src -I benchmarks $< $(LDFLAGS) -o $@
+	$(CXX) -std=c++11 $(CXXFLAGS) -DNDEBUG -O3 -flto -I src -I benchmarks $< $(LDFLAGS) -o $@
 	./json_benchmarks