Use install-sh.
# Use system-ac.mk if it exist, system.mk otherwise.
ifndef TOPDIR
TOPDIR=.
endif
ifeq ($(TOPDIR)/system-ac.mk,$(wildcard $(TOPDIR)/system-ac.mk))
# Using system-ac.mk
include $(TOPDIR)/system-ac.mk
else
# Not using system-ac.mk
include $(TOPDIR)/system.mk
endif