`static_linkage` is about how we link with dependency libraries.
We must set `LIBZIM_EXPORT_DLL` depending of how we build libzim library.

While they are internal (and not part of public API), unittest are testing
them and so we need to link to them.

On Windows, static members are not exported in dll.
So we must not use them in inlined method/constructor.

On Windows, if we declare a exported class, the implementation of this
class is expected in the dll.

When compiling without xapian we must not include headers for which we
don't compile the implementation.

MSVC throw a lot of warning because we also must re-export stl symbols
used in our exported class.

See [1] and [2] (and links inside) for more information.

Especially [3] which seems to say that we should have issue at link time if
it compatibility issue occurs (and so easily catchable).

So I just remove the `werror=true` for now.

[1] https://stackoverflow.com/questions/16419318/one-way-of-eliminating-c4251-warning-when-using-stl-classes-in-the-dll-interface
[2] https://stackoverflow.com/questions/2132747/warning-c4251-when-building-a-dll-that-exports-a-class-containing-an-atlcstrin
[3] https://stackoverflow.com/questions/2132747/warning-c4251-when-building-a-dll-that-exports-a-class-containing-an-atlcstrin#comment66255284_4563701