piwiki.js

var tracker = Piwik.getTracker( '{{PIWIK_URL}}', '1' );
var info = tracker.getVisitorInfo();
var optin = false;
var alreadyTracking = info[0] != '1' || info[3] > 0;

if (!alreadytracking) {
    optin = confirm( 'Opt-in to use cookies?' );
}

if (optin || alreadyTracking) {
    tracker.enableLinkTracking();
    tracker.trackPageView();
    // etc
}