[PATCH 1/5] include: Fix undefined char16_t with -DWINE_UNICODE_CHAR16.

[Puetz Kevin A]

> > We'd then typedef WCHAR to unsigned short, like we usually do. uchar.h
> > uses unsigned short as well, so the result should be the same without
> > introducing an additional includes.
> 
> That could work too, especially since in C char16_t is not a distinct type (and
> there's no function-overload mangling). Once you have <uchar.h>, char16_t
> is "the same type as uint16_t", which is almost certainly unsigned short
> anyway.
> So it's just a different route to the same result. And, since u"" *is* a built-in,
> one could even still offer TEXT macros for C11 in the final #else.
> 
> e.g. a shape like the following
> 
> #if defined(WINE_UNICODE_NATIVE)
> 	typedef wchar_t WCHAR
> 	#define TEXT(x) L ## x
> #elif defined(WINE_UNICODE_CHAR16) && defined(__cplusplus)
> 	typedef char16_t WCHAR
> 	#define TEXT(x) u ## x
> #else
> 	typedef unsigned short WCHAR
> 	#if __STDC_UTF_16__ /* C11 */
> 		#define TEXT(x) u ## x
> 	#endif
> #endif
> 
> It's a little less more complex (since you need to know how char16_t Is
> defined to argue that u"" matches this WCHAR), but I think it would work for
> any compiler that actually has a 16-bit unsigned integral type, and anything so
> exotic it doesn't can't really plausibly support wine.

Would you prefer that I redo this patch in the above shape? I think it would work,
but I personally prefer it as it currently stands (consistently using char16_t in both
C and C++). And I think there is sufficient opt-in; if you're  on a pre-C11 platform 
lacking <uchar.h>, or you don’t want any extra utf16 symbols in scope,
just don't define WINE_UNICODE_CHAR16.

Another option would be using __CHAR16_TYPE__, though that's gcc-specific.
I originally had a version of "include: Fix conflicting definitions of wchar_t..."
that used __WCHAR_TYPE__ in this fashion, before I settled on using <stddef.h> as 
a more portable solution and a path to also fixing my other ODR issue with NULL
(e.g. windef.h's 0 is 32-bit, gcc <stddef.h> uses __null, which can be a 64-bit type).