--no-xlib: prevent use of XInitThreads() and hence Xlib

--no-xlib: prevent use of XInitThreads() and hence Xlib

Certain LibVLC applications fail to call XInitThreads() before they
call XOpenDisplay(). Then VLC calls XInitThreads(). Then the
applications call XCloseDisplay(), which raises a segmentation fault.
In this case, Xlib tries acquire lock that was never created as the
Display ppinter was created before threaded Xlib mode was enabled.

These applications can now pass --no-xlib to libvlc_new(). This will
prevent any VLC Xlib-based plugin from being used. Currently, this
affects interfaces (not really an issue), PulseAudio (until Colin's
patch is merged upstream) and GLX (Xlib-based by design).

This will be necessary to address #3662.