Fixed Bug of not working on XP anymore... now only uses GetThreadId, if SDK supports it

Main/StackWalker/StackWalker.cpp

@@ -1032,9 +1032,13 @@ BOOL StackWalker::ShowCallstack(HANDLE hThread, const CONTEXT *context, PReadPro
   if (context == NULL)
   {
     // If no context is provided, capture the context
-    // if (hThread == GetCurrentThread())
     // See: https://stackwalker.codeplex.com/discussions/446958
+#if _WIN32_WINNT <= 0x0501
+      // If we need to support XP, we need to use the "old way", because "GetThreadId" is not available!
+    if (hThread == GetCurrentThread())
+#else
     if (GetThreadId(hThread) == GetCurrentThreadId())
+#endif
     {
       GET_CURRENT_CONTEXT_STACKWALKER_CODEPLEX(c, USED_CONTEXT_FLAGS);
     }