diff --git a/arch/sandbox/cpu/Makefile b/arch/sandbox/cpu/Makefile
index c5f5426..63deded 100644
--- a/arch/sandbox/cpu/Makefile
+++ b/arch/sandbox/cpu/Makefile
@@ -10,7 +10,10 @@
 obj-y	:= cpu.o os.o start.o state.o
 
 # os.c is build in the system environment, so needs standard includes
-$(obj)/os.o: CFLAGS := $(filter-out -nostdinc,\
-	$(patsubst -I%,-idirafter%,$(CFLAGS)))
-$(obj)/.depend.os: CPPFLAGS := $(filter-out -nostdinc,\
-	$(patsubst -I%,-idirafter%,$(CPPFLAGS)))
+# CFLAGS_REMOVE_os.o cannot be used to drop header include path
+quiet_cmd_cc_os.o = CC $(quiet_modtag)  $@
+cmd_cc_os.o = $(CC) $(filter-out -nostdinc, \
+	$(patsubst -I%,-idirafter%,$(c_flags))) -c -o $@ $<
+
+$(obj)/os.o: $(src)/os.c FORCE
+	$(call if_changed_dep,cc_os.o)