Add missing guard for __builtin_constant_p

Oversight in commit e2809e3a1. While at it, use pg_integer_constant_p
in master.

Discussion: https://postgr.es/m/CANWCAZbOha-x5MCreQn3TRA56VdKWNMAKMy3fAV1kJSw9Vp4pw@mail.gmail.com
Backpatch-through: 18

src/include/port/pg_crc32c.h

@@ -70,6 +70,7 @@ static inline
 pg_crc32c
 pg_comp_crc32c_dispatch(pg_crc32c crc, const void *data, size_t len)
 {
+#ifdef HAVE__BUILTIN_CONSTANT_P
 	if (__builtin_constant_p(len) && len < 32)
 	{
 		const unsigned char *p = (const unsigned char *) data;
@@ -91,6 +92,9 @@ pg_comp_crc32c_dispatch(pg_crc32c crc, const void *data, size_t len)
 	else
 		/* Otherwise, use a runtime check for AVX-512 instructions. */
 		return pg_comp_crc32c(crc, data, len);
+#else
+	return pg_comp_crc32c(crc, data, len);
+#endif							/* HAVE__BUILTIN_CONSTANT_P */
 }
 
 #elif defined(USE_SSE42_CRC32C_WITH_RUNTIME_CHECK)