Add HB_FALLTHROUGH
Borrowed from https://bugzilla.mozilla.org/show_bug.cgi?id=1215411
diff --git a/src/hb-private.hh b/src/hb-private.hh
index be29391..9db19eb 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -119,6 +119,36 @@
#define HB_FUNC __func__
#endif
+/*
+ * Borrowed from https://bugzilla.mozilla.org/show_bug.cgi?id=1215411
+ * HB_FALLTHROUGH is an annotation to suppress compiler warnings about switch
+ * cases that fall through without a break or return statement. HB_FALLTHROUGH
+ * is only needed on cases that have code:
+ *
+ * switch (foo) {
+ * case 1: // These cases have no code. No fallthrough annotations are needed.
+ * case 2:
+ * case 3:
+ * foo = 4; // This case has code, so a fallthrough annotation is needed:
+ * HB_FALLTHROUGH;
+ * default:
+ * return foo;
+ * }
+ */
+#if defined(__clang__) && __cplusplus >= 201103L
+ /* clang's fallthrough annotations are only available starting in C++11. */
+# define HB_FALLTHROUGH [[clang::fallthrough]]
+#elif defined(_MSC_VER)
+ /*
+ * MSVC's __fallthrough annotations are checked by /analyze (Code Analysis):
+ * https://msdn.microsoft.com/en-us/library/ms235402%28VS.80%29.aspx
+ */
+# include <sal.h>
+# define HB_FALLTHROUGH __fallthrough
+#else
+# define HB_FALLTHROUGH /* FALLTHROUGH */
+#endif
+
#if defined(_WIN32) || defined(__CYGWIN__)
/* We need Windows Vista for both Uniscribe backend and for
* MemoryBarrier. We don't support compiling on Windows XP,
