diff --git a/Makefile b/Makefile
index 59554e1d..57f19da3 100644
--- a/Makefile
+++ b/Makefile
@@ -18,8 +18,8 @@ endif
 OBJ := $(SRC:.c=.o)
 
 # define default flags, and override to append mandatory flags
-CFLAGS ?= -O3 -Wall -Wextra -Werror -Wshadow
-override CFLAGS += -std=gnu99 -fPIC -Ilib/src -Ilib/include
+override CFLAGS := -O3 -std=gnu99 -fPIC -Wall -Wextra -Werror -Wshadow $(CFLAGS)
+override CFLAGS += -Ilib/src -Ilib/include
 
 # ABI versioning
 SONAME_MAJOR := 0