convert static const ints to an enumerator

namespace LAMMPS_NS {

namespace Suffix {

  static const int NONE = 0;

  static const int OPT  = 1<<0;

  static const int GPU  = 1<<1;

  static const int OMP  = 1<<2;

  static const int INTEL  = 1<<3;

}

enum Suffix {

  NONE   = 0,

  OPT    = 1<<0,

  GPU    = 1<<1,

  OMP    = 1<<2,

  INTEL  = 1<<3,

  KOKKOS = 1<<4

};

}