Makefile: Do not overwrite existing mirror.list (fixes #18).

Makefile

@@ -11,7 +11,7 @@ install:
 	install -m 755 -D apt-mirror $(DESTDIR)$(PREFIX)/bin/apt-mirror
 	mkdir -p $(DESTDIR)$(PREFIX)/share/man/man1/
 	pod2man apt-mirror > $(DESTDIR)$(PREFIX)/share/man/man1/apt-mirror.1
-	install -m 644 -D mirror.list $(DESTDIR)/etc/apt/mirror.list
+	if test ! -f $(DESTDIR)/etc/apt/mirror.list; then install -m 644 -D mirror.list $(DESTDIR)/etc/apt/mirror.list; fi
 	mkdir -p $(DESTDIR)$(BASE_PATH)/mirror
 	mkdir -p $(DESTDIR)$(BASE_PATH)/skel
 	mkdir -p $(DESTDIR)$(BASE_PATH)/var