diff --git a/lib/libzstd.mk b/lib/libzstd.mk
index af12daff..c04957c6 100644
--- a/lib/libzstd.mk
+++ b/lib/libzstd.mk
@@ -79,7 +79,7 @@ endif
   CFLAGS += -fno-stack-protector -fomit-frame-pointer -fno-ident \
             -DDYNAMIC_BMI2=0 -DNDEBUG
 else
-  CFLAGS += -O3
+  CFLAGS ?= -O3
 endif
 
 DEBUGLEVEL ?= 0