Use _WIN32 instead of WIN32 to detect Windows

`WIN32` is not defined when building a 64-bit executable on Windows with Clang. `_WIN32` is the more appropriate option here.
http://web.archive.org/web/20191012035921/http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system contains a handy table/matrix of the options and why this is best.

rtgui/threadutils.cc

@@ -21,7 +21,7 @@
 #include <csignal>
 #include <iostream>
 
-#ifdef WIN32
+#ifdef _WIN32
 #include <windows.h>
 #endif
 
@@ -34,7 +34,7 @@ void MyMutex::checkLock ()
     if (locked) {
         std::cerr << "MyMutex already locked!" << std::endl;
 
-#ifdef WIN32
+#ifdef _WIN32
         DebugBreak ();
 #else
         raise (SIGTRAP);
@@ -49,7 +49,7 @@ void MyMutex::checkUnlock ()
     if (!locked) {
         std::cerr << "MyMutex already unlocked!" << std::endl;
 
-#ifdef WIN32
+#ifdef _WIN32
         DebugBreak ();
 #else
         raise (SIGTRAP);