Provide libvlc static build

As of 3.1, libvlc.so dynamically links with libc++_shared.so which is an issue for me because my users may use other libraries alongside libvlc which themselves require a different libc++_shared lib.

What I suggest we do:

• Modify the build script to add an option to build the libvlc statically.
• Optionally, set up the CI to make that build (though no point if it only interests me).

Context:

• 6ba71f4f
• b775dbba
• b9df5331