flambda-backend: Fix systhreads build

Author: mshinwell

otherlibs/systhreads/Makefile

@@ -61,7 +61,7 @@ allopt: lib$(LIBNAME)nat.$(A) $(LIBNAME).cmxa $(CMIFILES)
 lib$(LIBNAME).$(A): $(BYTECODE_C_OBJS)
 	$(V_OCAMLMKLIB)$(MKLIB_CMD) -o $(LIBNAME) $(BYTECODE_C_OBJS)
 
-lib$(LIBNAME)nat.$(A): OC_CFLAGS += $(OC_NATIVE_CFLAGS)
+lib$(LIBNAME)nat.$(A): OC_CFLAGS += $(OC_NATIVE_CFLAGS) -DNATIVE_CODE
 
 lib$(LIBNAME)nat.$(A): $(NATIVECODE_C_OBJS)
 	$(V_OCAMLMKLIB)$(MKLIB_CMD) -o $(LIBNAME)nat $^