Do not test WIN32, instead test _WIN32

[DimitriPapadopoulos]

To the best of my knowledge:

• _WIN32 and _WIN64 are defined by the compiler,
• WIN32 and WIN64 are defined by the user, to indicate whatever the user chooses them to indicate. They mean 32-bit and 64-bit Windows compilation by convention only.

See:
https://accu.org/journals/overload/24/132/wilson_2223/

Windows compilers in general, and MSVC in particular, have been defining _WIN32 and _WIN64 for a long time, provably at least since Visual Studio 2015, and in practice as early as in the days of 16-bit Windows.

See:
https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-140
https://learn.microsoft.com/en-us/windows/win32/winprog64/the-tools

Tests used to be inconsistent, sometimes testing _WIN32, sometimes _WIN32 and WIN32. This brings consistency to Windows detection.

zstd/zlibWrapper/gzguts.h

Lines 39 to 43 in cdceb0f

	#ifdef _WIN32
	# include <stddef.h>
	#else
	# include <unistd.h>
	#endif

zstd/programs/util.h

Lines 46 to 47 in cdceb0f

	#if defined(_WIN32)
	# include <windows.h>

zstd/programs/util.h

Lines 323 to 325 in cdceb0f

	#ifdef _WIN32
	# define UTIL_HAS_CREATEFILELIST
	#elif defined(__linux__) || (PLATFORM_POSIX_VERSION >= 200112L) /* opendir, readdir require POSIX.1-2001 */

zstd/contrib/pzstd/Options.cpp

Lines 63 to 67 in cdceb0f

	#ifdef _WIN32
	const char nullOutput[] = "nul";
	#else
	const char nullOutput[] = "/dev/null";
	#endif

zstd/lib/legacy/zstd_v07.h

Lines 27 to 31 in cdceb0f

	#if defined(_WIN32) && defined(ZSTDv07_DLL_EXPORT) && (ZSTDv07_DLL_EXPORT==1)
	# define ZSTDLIBv07_API __declspec(dllexport)
	#else
	# define ZSTDLIBv07_API
	#endif