Support both WP & WPMU

Hi!

I’m currently building a plugin for others to use to comunicate with us. The problem is that I want to support every blog even if it is in WP or WPMU and don’t know anyway around it.

(Every blog must have one uniq api-key)

global $blog_id;

//this will support WPMU but not WP
add_blog_option($blog_id, 'API_key', $post['api_key']);

/* it will throw error:
Fatal error: Call to undefined function get_blog_option() in my_plugin.php on line 61
...if WPMU is not enable
*/

//whould it be bad idéa if i used blog_id as an prefix?
//it works on both WP and WPMU
add_site_option('API_key'.$blog_id, $post['api_key']);