*: 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/decoder/plugins/FlacIOHandle.cxx

@@ -46,7 +46,7 @@ FlacIORead(void *ptr, size_t size, size_t nmemb, FLAC__IOHandle handle)
 
 			p += nbytes;
 
-#ifndef WIN32
+#ifndef _WIN32
 		} catch (const std::system_error &e) {
 			errno = e.code().category() == ErrnoCategory()
 				? e.code().value()