The MSVC /ZW flag is valid only for C++ sources. Whenever we enable
CompileAsWinRT for the whole target, disable it for all C sources.
Update the documentation of VS_WINRT_COMPONENT to drop the statement
about undefined behavior for non-C++ sources, because it is now
defined as expected.
Deprecate VS_WINRT_EXTENSIONS and document VS_WINRT_COMPONENT as for VS
generators only. Also define _WINRT_DLL in SHARED libraries in order to
get a .lib produced.
Inspired-by: Paul Annetts <paul@lightunobscured.com>