CL-USER> (defvar *debug-mode-p* t)
*DEBUG-MODE-P*

CL-USER> (defmacro with-debug (&body body)
           (when *debug-mode-p* `(progn ,@body)))
WITH-DEBUG

CL-USER> (defun foo ()
           (with-debug 
             "debug mode!"))
FOO

CL-USER> (foo)
"debug mode!"

CL-USER> (setf *debug-mode-p* nil)
NIL

CL-USER> (defun foo ()
           (with-debug 
             "debug mode!"))
WARNING: redefining COMMON-LISP-USER::FOO in DEFUN
FOO

CL-USER> (foo)
NIL