THRIFT-3206 Fix Visual Studio build failure due 'pthread_self': identifier not found

Patch:  Paweł Janicki
diff --git a/lib/cpp/src/thrift/transport/TSSLSocket.cpp b/lib/cpp/src/thrift/transport/TSSLSocket.cpp
index 6e9a4de..8e56426 100644
--- a/lib/cpp/src/thrift/transport/TSSLSocket.cpp
+++ b/lib/cpp/src/thrift/transport/TSSLSocket.cpp
@@ -74,7 +74,11 @@
 
 #if (OPENSSL_VERSION_NUMBER < OPENSSL_VERSION_NO_THREAD_ID)
 static unsigned long callbackThreadID() {
+#ifdef _WIN32
+  return (unsigned long)GetCurrentThreadId();
+#else
   return (unsigned long)pthread_self();
+#endif
 }
 #endif