use EIGEN_CPLUSPLUS instead of checking cpp version

Eigen/src/Core/StlIterators.h

diff --git a/Eigen/src/Core/StlIterators.h b/Eigen/src/Core/StlIterators.h
index bb897f8..25d4575 100644
--- a/Eigen/src/Core/StlIterators.h
+++ b/Eigen/src/Core/StlIterators.h
@@ -325,7 +325,7 @@
  public:
   typedef Index difference_type;
   typedef typename XprType::Scalar value_type;
-#if __cplusplus >= 202002L
+#if EIGEN_CPLUSPLUS >= 202002L
   typedef std::conditional_t<XprType::InnerStrideAtCompileTime == 1, std::contiguous_iterator_tag,
                              std::random_access_iterator_tag>
       iterator_category;