Use += to add to existing variable CXXFLAGS

Author: mhucka

tests/Makefile

@@ -40,7 +40,7 @@ endif
 
 CXX_FILES := $(BASIC_FILES) $(SSE_FILES) $(AVX2_FILES) $(AVX512_FILES)
 CXX_TARGETS := $(CXX_FILES:%.cc=%.x)
-CXXFLAGS := $(CXXFLAGS) $(SSE_FLAGS) $(AVX2_FLAGS) $(AVX512_FLAGS) $(BMI2_FLAGS)
+CXXFLAGS += $(SSE_FLAGS) $(AVX2_FLAGS) $(AVX512_FLAGS) $(BMI2_FLAGS)
 
 CUDA_FILES := $(wildcard *cuda_test.cu)
 CUDA_TARGETS := $(CUDA_FILES:%cuda_test.cu=%cuda_test.x)