-override CFLAGS := $(CFLAGS) $(WARN_FLAGS) $(extra_cflags)
-override CXXFLAGS := $(CXXFLAGS) $(WARN_FLAGS) $(extra_cflags) $(extra_cxxflags)
+override CFLAGS += $(WARN_FLAGS) $(extra_cflags)
+override CXXFLAGS += $(WARN_FLAGS) $(extra_cflags) $(extra_cxxflags)