*: 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/thread/Util.cxx

@@ -34,7 +34,7 @@
 #include <sched.h>
 #include <sys/syscall.h>
 #include <unistd.h>
-#elif defined(WIN32)
+#elif defined(_WIN32)
 #include <windows.h>
 #endif
 
@@ -71,7 +71,7 @@ SetThreadIdlePriority()
 
 	ioprio_set_idle();
 
-#elif defined(WIN32)
+#elif defined(_WIN32)
 	SetThreadPriority(GetCurrentThread(), THREAD_PRIORITY_IDLE);
 #endif
 };