I'm using a custom python program to display x11 idle time. I've set org-clock-x11idle-program-name accordingly but org-x11idle-exists-p is still nil. I'm guessing it's defvar'ed in org-clock.el before org-clock-x11idle-program-name is set. Evaluating the defvar of org-x11idle-exists-p afterwards indeed sets it to t.
I could do (setq org-x11idle-exists-p t) but there must be a better way to handle the definition of a new org-clock-x11idle-program-name. -- Julien Cubizolles