add check if CMB_PATH and CMB_URL are already defined

allows the user to simply include a custom value in their functions.php in case it is being included in a symlinked directory, e.g. (in fuctions.php before require_once call):
define( 'CMB_URL' , get_template_directory_uri() . '/library/Custom-Meta-Boxes/' );

this example returns the correct URI instead of something like http://base.uri/system/path/to/symlinked/folder/, which occurs when the system path to the wp_content_dir is different from the system path to the symlinked, e.g., theme folder.