[CMSP-643] Defining WP_HOME and WP_SITEURL in wp-config.php causes PHP notice

[marfillaster]

There are use-cases when WP_HOME and WP_SITEURL need to be derived from an alternative header other than HTTP_HOST. These constants are already being defined in wp-config-pantheon.php. Defining these constants before or after include 'wp-config-pantheon.php'; in wp-config.php will cause PHP notice. Since wp-config-pantheon.php is upstream maintained, we do not recommend modifying the file due to potential merge conflict in the future. Including a modified copy of wp-config-pantheon.php is what we suggest but any future update will require user to manually reconcile the files.

Proposed solution: Make the define statement in wp-config-pantheon.php conditional.