diff --git a/Makefile b/Makefile
index 6177c136c..05f73a54d 100644
--- a/Makefile
+++ b/Makefile
@@ -131,11 +131,14 @@ endif
 # Figure out optimize level.
 ifneq ($(DEBUG_LEVEL), 2)
 ifeq ($(LITE), 0)
-	OPT += -O2
+	OPTIMIZE_LEVEL ?= -O2
 else
-	OPT += -Os
+	OPTIMIZE_LEVEL ?= -Os
 endif
 endif
+# `OPTIMIZE_LEVEL` is empty when the user does not set it and `DEBUG_LEVEL=2`.
+# In that case, the compiler default (`-O0` for gcc and clang) will be used.
+OPT += $(OPTIMIZE_LEVEL)
 
 # compile with -O2 if debug level is not 2
 ifneq ($(DEBUG_LEVEL), 2)