gd_info - check if GD extesion is available

We have an environment where the GD extension is not installed because Imagick is used for image processing. In the webp.install file, there is a check to see if GD supports the WEBP format, but this check fails if GD is not installed at all, causing 500 error.

Error: Call to undefined function gd_info() v webp_requirements() /var/www/web/modules/contrib/webp/webp.install).