function abc() {
    $(function() {
        // do sth
    })
}

function addJQuery(callback) {
    "use strict";
    var script = document.createElement("script");
    script.textContent = "(" + callback.toString() + ")();";
    document.body.appendChild(script);
}
if (typeof $ === 'undefined') {
    if (unsafeWindow.jQuery) {
        $ = unsafeWindow.jQuery;
        abc();
    } else {addJQuery(abc);}
} else {
    abc();
}