Don't allow CFLAGS to be set from commandline.
Some build systems try to override CFLAGS, but this project needs it
to be set explicitly.
diff --git a/Makefile b/Makefile
index 63b2f51..16d3882 100644
--- a/Makefile
+++ b/Makefile
@@ -27,7 +27,7 @@
COMMONCFLAGS += $(call cc-option,$(CC),-fno-stack-protector,)
COMMONCFLAGS += $(call cc-option,$(CC),-fno-stack-protector-all,)
-CFLAGS = $(COMMONCFLAGS) -g -DMODE16=0
+override CFLAGS = $(COMMONCFLAGS) -g -DMODE16=0
CFLAGS16INC = $(COMMONCFLAGS) -DMODE16=1 -fno-jump-tables -fno-defer-pop \
$(call cc-option,$(CC),--param large-stack-frame=8,)
CFLAGS16 = $(CFLAGS16INC) -g