Fix issue #1968. Don't discard return value from "new" in C++17.

Eigen/src/Core/util/Memory.h

diff --git a/Eigen/src/Core/util/Memory.h b/Eigen/src/Core/util/Memory.h
index f1d1d0f..0b9589c 100644
--- a/Eigen/src/Core/util/Memory.h
+++ b/Eigen/src/Core/util/Memory.h
@@ -82,7 +82,7 @@
     //
     new int[huge];
     #else
-    ::operator new(huge);
+    void* unused = ::operator new(huge);
     #endif
   #endif
 }