sparse module: add preliminary support for direct sparse LU solver

using SuperLU. Calling SuperLU was very painful, but it was worth it,
it seems to be damn fast !

Eigen/Sparse

@@ -29,6 +29,27 @@
 
 #endif
 
+#ifdef EIGEN_SUPERLU_SUPPORT
+  typedef int int_t;
+  #include "superlu/slu_Cnames.h"
+  #include "superlu/supermatrix.h"
+  #include "superlu/slu_util.h"
+
+  namespace SuperLU_S {
+  #include "superlu/slu_sdefs.h"
+  }
+  namespace SuperLU_D {
+  #include "superlu/slu_ddefs.h"
+  }
+  namespace SuperLU_C {
+  #include "superlu/slu_cdefs.h"
+  }
+  namespace SuperLU_Z {
+  #include "superlu/slu_zdefs.h"
+  }
+  namespace Eigen { struct SluMatrix; }
+#endif
+
 namespace Eigen {
 
 #include "src/Sparse/SparseUtil.h"
@@ -54,6 +75,10 @@ namespace Eigen {
 # include "src/Sparse/TaucsSupport.h"
 #endif
 
+#ifdef EIGEN_SUPERLU_SUPPORT
+# include "src/Sparse/SuperLUSupport.h"
+#endif
+
 } // namespace Eigen
 
 #endif // EIGEN_SPARSE_MODULE_H