Import dependencies from VCPKG by default on Windows.

[emptyVoid]

Here's a proposal to import uncustomised dependency libraries from VCPKG instead of building them from sources. Importing is enabled by default on Windows and could be enabled via USE_VCPKG_* options on other platforms, the same options could be used to disable importing on Windows.

Resolves #866.

[JonnyH]

I'm unsure if this is actually needed going forward - we need the sources for other platforms already, and I personally like having sources locally to aid debugging into libraries - maybe there's something wrong with my setup but it doesn't seem to do source-level debugging with vcpkg-imported libraries.

[emptyVoid]

Have you tried updating vcpkg?
I've just tested stepping into pugi::xml_document::load_file -- works as expected.