This is not compatible with wpvip

[lazardanlucian]

can't activate plugin on WpVip, Automattic's own platform because of this!

Automattic's hosting WpVip GO is using advanced caching mechanisms and doesn't allow/expose the database
directly, only throught the wpsql api wpdb class.

I searched through the plugin and the plugin does not need DB_NAME,
only some test-scripts:

bin-wp/install-wp-tests.sh:9: DB_NAME=$1
bin-wp/install-wp-tests.sh:96: sed $ioption "s/youremptytestdbnamehere/$DB_NAME/" "$WP_TESTS_DIR"/wp-tests-config.php
bin-wp/install-wp-tests.sh:122: mysqladmin create $DB_NAME --user="$DB_USER" --password="$DB_PASS"$EXTRA

I'm not sure what the .sh are for, I can only hope they are for development purposes, because otherwise they might not work on many dockerised/contarinerised systems, etc.

Anyway, I'm pretty sure you don't need the:
if ( ! defined( 'DB_NAME' ) ) { header( 'HTTP/1.0 403 Forbidden' ); die; }