gd - function imagewebp doesn't exist in PHP 5.6? -

php-file:

<?php echo phpversion(); echo "\n"; var_dump(function_exists('imagewebp')); 

output:

5.6.0rc2 bool(false) 

gd support enabled (gd version:bundled (2.1.0 compatible))

---

where problem? "imagewebp" should exist since php 5.5.0 (see: http://php.net/manual/en/function.imagewebp.php)

ps: (i'm not one... how compile php enable webp support?)