librhash: Use __builtin_bswap{32,64} on Clang

Utilities/cmlibrhash/librhash/byte_order.h

@@ -85,6 +85,10 @@ void rhash_swap_copy_str_to_u64(void* to, int index, const void* from, size_t le
 void rhash_swap_copy_u64_to_str(void* to, const void* from, size_t length);
 void rhash_u32_mem_swap(unsigned *p, int length_in_u32);
 
+#ifndef __has_builtin
+# define __has_builtin(x) 0
+#endif
+
 /* define bswap_32 */
 #if defined(__GNUC__) && defined(CPU_IA32) && !defined(__i386__)
 /* for intel x86 CPU */
@@ -95,6 +99,8 @@ static inline uint32_t bswap_32(uint32_t x) {
 #elif defined(__GNUC__)  && (__GNUC__ >= 4) && (__GNUC__ > 4 || __GNUC_MINOR__ >= 3)
 /* for GCC >= 4.3 */
 # define bswap_32(x) __builtin_bswap32(x)
+#elif defined(__clang__) && __has_builtin(__builtin_bswap32)
+# define bswap_32(x) __builtin_bswap32(x)
 #elif (_MSC_VER > 1300) && (defined(CPU_IA32) || defined(CPU_X64)) /* MS VC */
 # define bswap_32(x) _byteswap_ulong((unsigned long)x)
 #elif !defined(__STRICT_ANSI__)
@@ -110,6 +116,8 @@ static inline uint32_t bswap_32(uint32_t x) {
 
 #if defined(__GNUC__) && (__GNUC__ >= 4) && (__GNUC__ > 4 || __GNUC_MINOR__ >= 3)
 # define bswap_64(x) __builtin_bswap64(x)
+#elif defined(__clang__) && __has_builtin(__builtin_bswap64)
+# define bswap_64(x) __builtin_bswap64(x)
 #elif (_MSC_VER > 1300) && (defined(CPU_IA32) || defined(CPU_X64)) /* MS VC */
 # define bswap_64(x) _byteswap_uint64((__int64)x)
 #elif !defined(__STRICT_ANSI__)