unix/Makefile: Fix regression using install on non-GNU systems.

This was fixed previously in 31fc81d3b8 but
regressed in 4af79e7694.

Fixes #5885.

ports/unix/Makefile

@@ -266,7 +266,8 @@ PREFIX = /usr/local
 BINDIR = $(DESTDIR)$(PREFIX)/bin
 
 install: $(PROG)
-	install -D $(PROG) $(BINDIR)/$(PROG)
+	install -d $(BINDIR)
+	install $(PROG) $(BINDIR)/$(PROG)
 
 uninstall:
 	-rm $(BINDIR)/$(PROG)