For Eigen 5 compatibility

When using Eigen 5, in FunctorsPlugin.hpp the macro EIGEN_EMPTY_STRUCT_CTOR has been remove
and for c++14,this macro is not needed.
So I suggest add macro for it like:

> #if (!defined(_MSC_VER) && __cplusplus < 201402L) || (defined(_MSC_VER) && (!defined(_MSVC_LANG) || _MSVC_LANG < 201402L))
>   EIGEN_EMPTY_STRUCT_CTOR(scalar_sum_of_finites_op)
> #endif 

or something similar