diff --git a/Makefile b/Makefile
index 974248b..75dacbc 100644
--- a/Makefile
+++ b/Makefile
@@ -39,8 +39,8 @@ GIT_VASM             := https://github.com/leffmann/vasm
 GIT_VBCC             := https://github.com/bebbo/vbcc
 GIT_VLINK            := https://github.com/leffmann/vlink
 
-CFLAGS := -Os
-CXXFLAGS := $(CFLAGS)
+CFLAGS ?= -Os
+CXXFLAGS ?= $(CFLAGS)
 CFLAGS_FOR_TARGET ?= -Os -fomit-frame-pointer
 CXXFLAGS_FOR_TARGET ?= $(CFLAGS_FOR_TARGET) -fno-exceptions -fno-rtti