Undefed Yield symbol if defined, else syntax error

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@1994 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/thread.cpp

@@ -440,6 +440,10 @@ bool wxThread::IsMain()
     return ::GetCurrentThreadId() == s_idMainThread;
 }
 
+#ifdef Yield
+#undef Yield
+#endif
+
 void wxThread::Yield()
 {
     // 0 argument to Sleep() is special