ParMETIS with `real_t != float` not supported

When compiling dune-istl with ParMETIS, I get the error

dune/istl/repartition.hh:1381:27: error: cannot convert 'float*' to 'real_t* {aka double*}' 
  for argument '10' to 'int ParMETIS_V3_PartKway'

Explanation: ParMETIS was compiled with double precision instead of single precision

Difficulty: I think, old versions of ParMETIS do not have a #define real_t ...

Solution: Use real_t* instead of float*, with additionally something like

#ifndef REALTYPEWIDTH
#define real_t float
#endif

This issue is relevant for the Dune 2.6 release.