Guard usage of decltype since it's a C++11 feature

This fixes https://gitlab.com/libeigen/eigen/-/issues/1897

test/indexed_view.cpp

@@ -439,11 +439,13 @@ void check_indexed_view()
     VERIFY( MATCH( A(all,1)(1), "101"));
   }
 
+#if EIGEN_HAS_CXX11
   //Bug IndexView with a single static row should be RowMajor:
   {
     // A(1, seq(0,2,1)).cwiseAbs().colwise().replicate(2).eval();
     STATIC_CHECK(( (internal::evaluator<decltype( A(1,seq(0,2,1)) )>::Flags & RowMajorBit) == RowMajorBit ));
   }
+#endif
 
 }