Fix fallthrough compiler warning with gcc >= 7
Fixes https://github.com/harfbuzz/harfbuzz/issues/755
diff --git a/src/hb-private.hh b/src/hb-private.hh
index eba797c..53ed11c 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -154,6 +154,9 @@
#if defined(__clang__) && __cplusplus >= 201103L
/* clang's fallthrough annotations are only available starting in C++11. */
# define HB_FALLTHROUGH [[clang::fallthrough]]
+#elif __GNUC__ >= 7
+ /* GNU fallthrough attribute is available from GCC7 */
+# define HB_FALLTHROUGH __attribute__((fallthrough))
#elif defined(_MSC_VER)
/*
* MSVC's __fallthrough annotations are checked by /analyze (Code Analysis):
