function linksextern() {
    var links = document.getElementsByTagName('a');
    var links;
    for (index in links) {
        if (links[index].nodeType === 1) {
            var result = links[index].getAttribute('href');
            if (result.substr(0, 5).match('http:') || result.substr(0, 6).match('https:')) {
                links[index].setAttribute('onclick', 'window.open(this.href); return false;');
            }
        }
    }
}

window.onload = linksextern;