Use __attribute__((unused)) only with gcc 4 and later

See discussion at:
https://bugzilla.gnome.org/show_bug.cgi?id=610183

src/hb-private.h

@@ -184,14 +184,17 @@ typedef int hb_mutex_t;
 #endif
 
 #if __GNUC__ >= 3
-#define HB_GNUC_UNUSED	__attribute__((unused))
 #define HB_GNUC_PURE	__attribute__((pure))
 #define HB_GNUC_CONST	__attribute__((const))
 #else
-#define HB_GNUC_UNUSED
 #define HB_GNUC_PURE
 #define HB_GNUC_CONST
 #endif
+#if __GNUC__ >= 4
+#define HB_GNUC_UNUSED	__attribute__((unused))
+#else
+#define HB_GNUC_UNUSED
+#endif
 
 
 #if (defined(__WIN32__) && !defined(__WINE__)) || defined(_MSC_VER)