Win32: use linked_with_a_crap_library_which_uses_atexit() (libxml2 and libqt4)

Win32: use linked_with_a_crap_library_which_uses_atexit() (libxml2 and libqt4)

  On windows,
  - libxml2 spawns a 'watchdog' thread, when statically linked.
  - libqt4 spawns a 'watcher' thread.

  These threads clean up remaining ressources once their initiator thread has
  terminated. So, the associated dll must NOT be freed explicitly and blindly.
  Since there is no means to know when these 'cleanup' threads terminate,
  the safe approach is to not free the dlls and let freeing occur when vlc
  terminates.