I had the same issue with a clean install. I save the plugin to the plugins folder /wp-content/plugins/memcached.
Adding if( !function_exists( ‘wp_cache_add’ ) ){ to line 16 and closing it after the last function declaration fixed the problem for me.
I also had to add if(! class_exists(‘WP_Object_Cache’) ) { right before the class declaration.
Probably not the final solution but it got it working for me.
Memcache plug version 3.0.1
Wordpress 4.7.3