Only build x11 plugin when we have Xext.h.

Now, UDP-Lite really works!

This is completely untested because vlc does not link at the moment :(

Work from DJ_Mulder on the forum, changes by /me helped by dionoea and jpsaman.

- added controlled symbol export support for win32. Now, all visible symbols in libvlc.dll must be declared using either VLC_EXPORT or VLC_PUBLIC_API. Otherwise, they will not be exported. this has the advantage of reducing the number of symbols in the DLL, saving on file size an binding time at runtime. All exported symbols will be listed in the libvlc.def file.
This also pave the way for using the visibility feature in GCC 4, adding controlled symbols export for UNIX/MacOS X as well.

   compatible with UNIX fortune executable.

    - removed <10.3 support in the preferences' code
    - added preliminary support for Obj-C 2.0 / Garbage Collection
        - needs to be enabled manually in intf.h for now and is restricted to future Darwin release
    - updated various copyright dates to approbate values
    - configured the source tree in the Xcode project for future Xcode releases
    - finally renamed the Xcode project to the correct bundle extension