*: check defined(_WIN32) instead of defined(WIN32)

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

src/util/StringAPI.hxx

@@ -105,7 +105,7 @@ gcc_nonnull_all
 static inline char *
 UnsafeCopyStringP(char *dest, const char *src) noexcept
 {
-#if defined(WIN32) || defined(__BIONIC__)
+#if defined(_WIN32) || defined(__BIONIC__)
   /* emulate stpcpy() */
   UnsafeCopyString(dest, src);
   return dest + StringLength(dest);