Issue an error in case of direct inclusion of internal headers.

Eigen/src/OrderingMethods/Amd.h

@@ -21,6 +21,8 @@ the Mozilla Public License v. 2.0, as stated at the top of this file.
 #ifndef EIGEN_SPARSE_AMD_H
 #define EIGEN_SPARSE_AMD_H
 
+#include "./InternalHeaderCheck.h"
+
 namespace Eigen { 
 
 namespace internal {

Eigen/src/OrderingMethods/InternalHeaderCheck.h

@@ -0,0 +1,3 @@
+#ifndef EIGEN_ORDERINGMETHODS_MODULE_H
+#error "Please include Eigen/OrderingMethods instead of including headers inside the src directory directly."
+#endif

Eigen/src/OrderingMethods/Ordering.h

@@ -11,6 +11,8 @@
 #ifndef EIGEN_ORDERING_H
 #define EIGEN_ORDERING_H
 
+#include "./InternalHeaderCheck.h"
+
 namespace Eigen {
   
 #include "Eigen_Colamd.h"