-KBUILD_CPPFLAGS += $(CPPFLAGS)
-KBUILD_AFLAGS += $(AFLAGS)
-KBUILD_CFLAGS += $(CFLAGS)
+# But warn user when we do so
+warn-assign = \
+$(warning "WARNING: Appending $$K$(1) ($(K$(1))) from $(origin K$(1)) to kernel $$$(1)")
+
+ifneq ($(KCPPFLAGS),)
+ $(call warn-assign,CPPFLAGS)
+ KBUILD_CPPFLAGS += $(KCPPFLAGS)
+endif
+ifneq ($(KAFLAGS),)
+ $(call warn-assign,AFLAGS)
+ KBUILD_AFLAGS += $(KAFLAGS)
+endif
+ifneq ($(KCFLAGS),)
+ $(call warn-assign,CFLAGS)
+ KBUILD_CFLAGS += $(KCFLAGS)
+endif