Ensure compatibility with multiple autoloaders

Running a .phar or regular PHP executable that requires and includes its own
version of php-parser will cause a "cannot redeclare class" error if said
executable also includes the autoloader of the current working directory.

lib/PhpParser/Autoloader.php

@@ -7,14 +7,25 @@ namespace PhpParser;
  */
 class Autoloader
 {
+    /**
+     * Whether the autoloader has been registered.
+     *
+     * @var boolean
+     */
+    protected static $registered = false;
+
     /**
      * Registers PhpParser\Autoloader as an SPL autoloader.
      *
      * @param bool $prepend Whether to prepend the autoloader instead of appending
      */
     static public function register($prepend = false) {
+        if (static::$registered === true) {
+            return;
+        }
         ini_set('unserialize_callback_func', 'spl_autoload_call');
         spl_autoload_register(array(__CLASS__, 'autoload'), true, $prepend);
+        static::$registered = true;
     }
 
     /**

lib/bootstrap.php

@@ -1,4 +1,6 @@
 <?php
 
-require __DIR__ . '/PhpParser/Autoloader.php';
+if (!class_exists('PhpParser\Autoloader')) {
+    require __DIR__ . '/PhpParser/Autoloader.php';
+}
 PhpParser\Autoloader::register();