Fix a portability issue for GCC < 3.4.0
diff --git a/timsort.h b/timsort.h
index b3a32f0..99697a0 100644
--- a/timsort.h
+++ b/timsort.h
@@ -48,7 +48,7 @@
 int compute_minrun(uint64_t);
 
 #ifndef CLZ
-#ifdef __GNUC__
+#if defined(__GNUC__) && ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__GNUC__ > 3))
 #define CLZ __builtin_clzll
 #else