Only call XInitThreads() when we have a very new X11 library. Checked by looking whether XKeycodeToKeysym() is deprecated, which also happend in some very new X11 version.
Fix for [3613668] by not calling XInitThreads when either 1) Tcl is compiled without threads 2) X11 library is too old.
A more direct test would be more desirable, but for now this should be fine.