diff --git a/arch/sandbox/cpu/Makefile b/arch/sandbox/cpu/Makefile
index e5e860b..2ae0f71 100644
--- a/arch/sandbox/cpu/Makefile
+++ b/arch/sandbox/cpu/Makefile
@@ -23,9 +23,6 @@
 # MA 02111-1307 USA
 #
 
-# os.c is build in the system environment, so needs standard includes
-CPPFLAGS_arch/sandbox/cpu/os.o += -I/usr/include
-
 include $(TOPDIR)/config.mk
 
 LIB	= $(obj)lib$(CPU).o
@@ -40,6 +37,10 @@
 $(LIB):	$(OBJS)
 	$(call cmd_link_o_target, $(OBJS))
 
+# os.c is build in the system environment, so needs standard includes
+$(obj)os.o: ALL_CFLAGS := $(filter-out -nostdinc,$(ALL_CFLAGS))
+$(obj).depend.os: CPPFLAGS := $(filter-out -nostdinc,$(CPPFLAGS))
+
 #########################################################################
 
 # defines $(obj).depend target