std::integral_constant is not C++03 compatible

Eigen/src/Core/util/XprHelper.h

@@ -36,14 +36,16 @@ inline IndexDest convert_index(const IndexSrc& idx) {
 
 // true if T can be considered as an integral index (i.e., and integral type or enum)
 template<typename T> struct is_valid_index_type
-  : std::integral_constant<bool,
+{
+  enum { value =
 #if EIGEN_HAS_TYPE_TRAITS
    internal::is_integral<T>::value || std::is_enum<T>::value
 #else
   // without C++11, we use is_convertible to Index instead of is_integral in order to treat enums as Index.
   internal::is_convertible<T,Index>::value
 #endif
-> {};
+  };
+};
 
 // promote_scalar_arg is an helper used in operation between an expression and a scalar, like:
 //    expression * scalar