[bugfix] "properly" from field_type to int

we have to retrieve an int which is stored in field_type. If the field_type is std::complex, we
have to access the real-component, if it is double/float, we don't have to do anything
special. We use std::real(.) to get the correct real value. This function (with its appropriate
overloads) is available from c++11 on, but this part was already supported by the ancient
g++ version we supprt ;-)

dune/istl/ilu.hh

@@ -188,7 +188,9 @@ namespace Dune {
           coliterator kj = ILU[(*ik).first].find((*ik).first);                       // diagonal in k
           for (++kj; kj!=endk; ++kj)                       // row k eliminates in row i
           {
-            int generation = (int) firstmatrixelement(*kj);
+            // we misuse the storage to store an int. If the field_type is std::complex, we have to access the real part
+            // starting from C++11, we can use std::real to always return a real value, even if it is double/float
+            int generation = (int) std::real( firstmatrixelement(*kj) );
             if (generation<n)
             {
               mapiterator ij = rowpattern.find(kj.index());