Fix org-clock evaluation startup hang on Windows

Fix org-clock evaluation startup hang on Windows

* lisp/org-clock.el (org-x11idle-exists-p): Only shell out when running
on X.

The definition of this variable currently executes "command" via the
shell during evaluation, irrespective of the platform on which
Org-mode is running. Unfortunately, on Windows, this matches the
"command.com" NT Virtual DOS Machine executable and so this gets
launched, but this is a shell and therefore sits there waiting for
user input and never returns. The net result is that Emacs will hang
on Windows when evaluating org-clock.el with the ntdvm.exe process
spinning at 100%.

The simple fix is to check that the platform is X before trying to
deal with the "x11idle" external process.

TINYCHANGE