- added controlled symbol export support for win32. Now, all visible symbols in libvl...

- 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.