Fix warnings about repeated deinitions of macros.

test/assignment_threaded.cpp

@@ -7,13 +7,10 @@
 // Public License v. 2.0. If a copy of the MPL was not distributed
 // with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-#if defined(EIGEN_USE_THREADS)
-#undef EIGEN_USE_THREADS
-#endif
-
-#define EIGEN_USE_THREADS
+#define EIGEN_USE_THREADS 1
 
 #include "main.h"
+#include <Eigen/ThreadPool>
 
 namespace Eigen {
 namespace internal {