Increase portability to more C++ compilers

src/suffix.h

@@ -16,15 +16,16 @@
 
 namespace LAMMPS_NS {
 
-enum Suffix {
+namespace Suffix {
+  enum {
     NONE   = 0,
     OPT    = 1<<0,
     GPU    = 1<<1,
     OMP    = 1<<2,
     INTEL  = 1<<3,
     KOKKOS = 1<<4
-};
-
+  };
+}
 }
 
 #endif