CMake detect Cray PE

Cray PE programming environment is a software environment for developing running applications on Cray supercomputers. Use the Cray compiler wrappers cc, CC, and ftn instead of directly referencing the compiler backends gcc, g++, and gfortran.

In CMake script, detect if Cray PE is being used, regardless of compiler backend by detecting if environment variable CRAYPE_VERSION is set.

  message("Cray PE detected")