Limit __builtin_bswap16 to GCC >= 5 as it was implemented on 4.8

src/hb-machinery.hh

@@ -721,7 +721,7 @@ struct BEInt<Type, 2>
   }
   inline operator Type (void) const
   {
-#if defined(__GNUC__) || defined(__clang__)
+#if (defined(__GNUC__) && __GNUC__ >= 5) || defined(__clang__)
     /* Spoon-feed the compiler a big-endian integer with alignment 1.
      * https://github.com/harfbuzz/harfbuzz/pull/1398 */
     struct __attribute__((packed)) packed_uint16_t { uint16_t v; };