--- a/makefile
+++ b/makefile
@@ -497,8 +497,8 @@ ifneq ($(DEBUG),)
   CFLAGS_O = -O0
   BUILD_FEATURES = - debugging support
 else
-  CFLAGS_O = -O2
-  LDFLAGS_O = 
+  CFLAGS_O ?= -O2
+  LDFLAGS_O ?= 
   ifeq (Darwin,$(OSTYPE))
     NO_LTO = 1
   endif