add EIGEN_FUNCTORS_PLUGIN

Eigen/src/Core/Functors.h

diff --git a/Eigen/src/Core/Functors.h b/Eigen/src/Core/Functors.h
index 347f1b2..c8ca3da 100644
--- a/Eigen/src/Core/Functors.h
+++ b/Eigen/src/Core/Functors.h
@@ -348,6 +348,12 @@
 struct ei_functor_traits<ei_scalar_identity_op<Scalar> >
 { enum { Cost = NumTraits<Scalar>::AddCost, PacketAccess = false, IsRepeatable = true }; };
 
+// allow to add new functors and specializations of ei_functor_traits from outside Eigen.
+// this macro is really needed because ei_functor_traits must be specialized after it is declared but before it is used...
+#ifdef EIGEN_FUNCTORS_PLUGIN
+#include EIGEN_FUNCTORS_PLUGIN
+#endif
+
 // all functors allow linear access, except ei_scalar_identity_op. So we fix here a quick meta
 // to indicate whether a functor allows linear access, just always answering 'yes' except for
 // ei_scalar_identity_op.