Build system does not honour $(DESTDIR)

--- a/Makefile
+++ b/Makefile
@@ -54,8 +54,8 @@
 uninstall: uninstall-headers uninstall-docs
 
 install-headers:
-	mkdir -p $(PREFIX)/include/png++
-	cp $(headers) $(PREFIX)/include/png++
+	mkdir -p $(DESTDIR)$(PREFIX)/include/png++
+	cp $(headers) $(DESTDIR)$(PREFIX)/include/png++
 
 uninstall-headers:
 	rm -rf $(PREFIX)/include/png++