use rsync to delete obsolete files

This deletes old and stale files, which are not included in the
generated content anymore. This keeps the target directory nice and
clean.

Makefile

@@ -20,7 +20,7 @@ clean:
 	-rm -r public/*
 
 install:
-	cp -r public/ $(HTTPDIR)/
+	rsync --delete -r public/ $(HTTPDIR)/
 	find $(HTTPDIR) ! -path $(HTTPDIR) -type d -exec chmod $(DIRMODE) {} +
 	find $(HTTPDIR) ! -path $(HTTPDIR) -type f -exec chmod $(FILEMODE) {} +
 	chown -R $(OWNER):$(GROUP) $(HTTPDIR)/