The VSS headers are part of standard MS VS SDK, at least since version
15, and probably before that.

They are also included with MinGW, although currently broken.

Let's streamline a bit the options, by not making it so special, and
instead rely on proper system headers configuration or user
--extra-cxxflags. This still requires some extra step to cross-build
with MinGW as described in the meson.build file now.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[Use a "feature"-type option. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>