Define HB_UNUSED for clang

Author: IceflowRE

src/hb.hh

@@ -241,7 +241,7 @@ extern "C" void  hb_free_impl(void *ptr);
 #define HB_CONST_FUNC
 #define HB_PRINTF_FUNC(format_idx, arg_idx)
 #endif
-#if defined(__GNUC__) && (__GNUC__ >= 4)
+#if defined(__GNUC__) && (__GNUC__ >= 4) || (__clang__)
 #define HB_UNUSED	__attribute__((unused))
 #elif defined(_MSC_VER) /* https://github.com/harfbuzz/harfbuzz/issues/635 */
 #define HB_UNUSED __pragma(warning(suppress: 4100 4101))