Noticed that the static lib define was never set, so the exports were wrong.

Not sure if KDSOAP_STATICLIB should also be set when using the lib, though.
Let's see what Windows says.
Small refactoring: add serializePart()
Omit non-nillable empty elements of a complex type.

This fixes the unwanted <n1:ParentFolderIds/> in the MS EWS request,
which is invalid according to the schema (if the element is specified,
it must have content).