suppress polluting EMPTY macro defined by SuperLU

unsupported/Eigen/SuperLUSupport

@@ -5,11 +5,27 @@
 
 #include "../../Eigen/src/Core/util/DisableStupidWarnings.h"
 
+#ifdef EMPTY
+#define EIGEN_EMPTY_WAS_ALREADY_DEFINED
+#endif
+
 typedef int int_t;
 #include <slu_Cnames.h>
 #include <supermatrix.h>
 #include <slu_util.h>
 
+// slu_util.h defines a preprocessor token named EMPTY which is really polluting,
+// so we remove it in favor of a SUPERLU_EMPTY token.
+// If EMPTY was already, defined then we don't undef it.
+
+#if defined(EIGEN_EMPTY_WAS_ALREADY_DEFINED)
+# undef EIGEN_EMPTY_WAS_ALREADY_DEFINED
+#elif defined(EMPTY)
+# undef EMPTY
+#endif
+
+#define SUPERLU_EMPTY (-1)
+
 namespace Eigen { struct SluMatrix; }
 
 namespace Eigen {
@@ -17,7 +33,7 @@ namespace Eigen {
 /** \ingroup Unsupported_modules
   * \defgroup SuperLUSupport_Module Super LU support
   *
-  *
+  * \warning When including this module, you have to use SUPERLU_EMPTY instead of EMPTY which is no longer defined because it is too polluting.
   *
   * \code
   * #include <Eigen/SuperLUSupport>