Add check to verify that enableClasses has been initialized.

During custom builds that exclude this functionality enableClasses is never initialized, causing errors to be raised on this line. This patch verifies that enableClasses is initialized before testing the value of enableClasses.

modernizr.js

@@ -979,7 +979,7 @@ window.Modernizr = (function( window, document, undefined ) {
 
          test = typeof test == 'function' ? test() : test;
 
-         if (enableClasses) {
+         if (typeof enableClasses !== "undefined" && enableClasses) {
            docElement.className += ' ' + (test ? '' : 'no-') + feature;
          }
          Modernizr[feature] = test;