Replace WIN32 with _WIN32

Using WIN32 depends on the build environment defining the variable.
_WIN32 is a predefined MSVC macro and is always available.

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

lib/init.c

@@ -21,7 +21,7 @@
 
 #define _GNU_SOURCE
 
-#if defined(WIN32)
+#if defined(_WIN32)
 #include "win32/win32_compat.h"
 #else
 #include <strings.h>