Fixes compilation with SuperLU 7 by forcing backwards compatibility

SuperLU 7.0.0 now uses singlecomplex instead of complex and by default
does not add a typedef to make singlecomplex available as type
complex. The latter was used in previous versions.

As we need to support several versions of SuperLU we force adding such
a typedef by defining SUPERLU_TYPEDEF_COMPLEX

dune/istl/superlufunctions.hh

@@ -58,6 +58,11 @@ extern "C" {
 #endif
 
 #if __has_include("slu_cdefs.h")
+#ifndef SUPERLU_TYPEDEF_COMPLEX
+// For now let's keep backwards compatible and let
+// complex be a typedef of doublecomplex
+#define SUPERLU_TYPEDEF_COMPLEX
+#endif
 #include "slu_scomplex.h"
 
 extern "C" {