Do not test WIN32, instead test _WIN32

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, usually testing `_WIN32` and here testing `WIN32`.
This brings consistency to Windows detection.

test/minigzip.c

@@ -40,7 +40,7 @@
 #  include <sys/stat.h>
 #endif
 
-#if defined(MSDOS) || defined(OS2) || defined(WIN32) || defined(__CYGWIN__)
+#if defined(MSDOS) || defined(OS2) || defined(_WIN32) || defined(__CYGWIN__)
 #  include <fcntl.h>
 #  include <io.h>
 #  ifdef UNDER_CE
@@ -67,7 +67,7 @@
 #endif
 
 #if !defined(Z_HAVE_UNISTD_H) && !defined(_LARGEFILE64_SOURCE)
-#ifndef WIN32 /* unlink already in stdio.h for WIN32 */
+#ifndef _WIN32 /* unlink already in stdio.h for _WIN32 */
   extern int unlink(const char *);
 #endif
 #endif