FIX: compilation failure when using XCode

src/include/reb-c.h

@@ -249,6 +249,11 @@ typedef void(*CFUNC)(void *);
 **
 ***********************************************************************/
 
+#ifdef __has_builtin
+#  define HAS_BUILTIN(x) __has_builtin(x)
+#else
+#  define HAS_BUILTIN(x) 0
+#endif
 
 #if defined(_MSC_VER)
 # define FORCE_INLINE    __forceinline
@@ -259,16 +264,21 @@ typedef void(*CFUNC)(void *);
 // Other compilers..
 #else   // defined(_MSC_VER)
 # define FORCE_INLINE inline __attribute__((always_inline))
-inline uint32_t rotl32(uint32_t x, int8_t r)
-{
-    return (x << r) | (x >> (32 - r));
-}
-inline uint64_t rotl64(uint64_t x, int8_t r)
-{
-    return (x << r) | (x >> (64 - r));
-}
-# define ROTL32(x,y) rotl32(x,y)
-# define ROTL64(x,y) rotl64(x,y)
+# if HAS_BUILTIN(__builtin_rotateleft32) && HAS_BUILTIN(__builtin_rotateleft64)
+#  define ROTL32(x,y) __builtin_rotateleft32(x,y)
+#  define ROTL64(x,y) __builtin_rotateleft64(x,y)
+# else
+    FORCE_INLINE uint32_t rotl32(uint32_t x, int8_t r)
+    {
+        return (x << r) | (x >> (32 - r));
+    }
+    FORCE_INLINE uint64_t rotl64(uint64_t x, int8_t r)
+    {
+        return (x << r) | (x >> (64 - r));
+    }
+#  define ROTL32(x,y) rotl32(x,y)
+#  define ROTL64(x,y) rotl64(x,y)
+# endif
 # define BIG_CONSTANT(x) (x##LLU)
 #endif // !defined(_MSC_VER)