The previous segfaults on OS X have been isolated to a problem in Cocoa Tk and an issue has been opened about it on the Tk issue tracker.  See Issue22770 for details.  Changesets applied to _is_gui_available() in that issue should prevent the segfaults by ensuring that Tk completes necessary initialization before the Tcl interpreter instance is destroyed and a new one created.  So _is_gui_available() is now enabled on OS X and we can close this issue.