diff --git a/hello_world/Makefile b/hello_world/Makefile
index ee4f5d8..e04d0c6 100644
--- a/hello_world/Makefile
+++ b/hello_world/Makefile
@@ -6,7 +6,7 @@
 
 .PHONY: all
 all:
-	$(MAKE) -C host CROSS_COMPILE="$(HOST_CROSS_COMPILE)"
+	$(MAKE) -C host CROSS_COMPILE="$(HOST_CROSS_COMPILE)" --no-builtin-variables
 	$(MAKE) -C ta CROSS_COMPILE="$(TA_CROSS_COMPILE)"
 
 .PHONY: clean
diff --git a/hello_world/host/Makefile b/hello_world/host/Makefile
index e3c68c6..c4c8239 100644
--- a/hello_world/host/Makefile
+++ b/hello_world/host/Makefile
@@ -23,3 +23,6 @@
 .PHONY: clean
 clean:
 	rm -f $(OBJS) $(BINARY)
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c $< -o $@