Remove CMake policy CMP0026
Remove CMake policy CMP0026

As no build target uses the LOCATION property, this policy doesn't have
to be set anymore. Recent CMake versions disallow setting this policy
to OLD.

RWTH-ELP/PnMPI,RWTH-HPC/PnMPI,RWTH-ELP/PnMPI,RWTH-ELP/PnMPI,RWTH-HPC/PnMPI,RWTH-HPC/PnMPI