macos/win32: add an option to select the (lib)VLC license

Some contribs may be disabled depending on the license chosen.

By default we build with GPLv3 compatibility. libvlc-only builds may want to restrict to LGPL v3 with ad-clause or LGPL v2.1 with ad-clause.

faad is automatically picked when building for GPL and not picked otherwise. We should never force it.

live555 should not be forced if the license doesn't allow it (win32).