logging.h: undefine DEBUG, ERROR, and WARN

These macros are erroneously defined by other subsystems, we undefine
them with a warning if they are already defined.

Change-Id: I97bed7d15d54f81fa72d1a0a598d9980c961692b
Reviewed-on: http://review.couchbase.org/37855
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>