message: Supress fallthrough warning for gcc 7.1.1
diff: #define REALM_HAS_CPP_ATTRIBUTE(attr) 0
  #endif
  -#if REALM_HAS_CPP_ATTRIBUTE(clang::fallthrough)
  +#if REALM_HAS_CPP_ATTRIBUTE(fallthrough)
  +#define REALM_FALLTHROUGH [[fallthrough]]
  +#elif REALM_HAS_CPP_ATTRIBUTE(clang::fallthrough)
  #define REALM_FALLTHROUGH [[clang::fallthrough]]
  #else
  #define REALM_FALLTHROUGH
  