Fix GCC7 fallthrough warnings

src/blackmisc/fallthrough.h

@@ -0,0 +1,33 @@
+/* Copyright (C) 2018
+ * swift project Community / Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution and at http://www.swift-project.org/license.html. No part of swift project,
+ * including this file, may be copied, modified, propagated, or distributed except according to the terms
+ * contained in the LICENSE file.
+ */
+
+//! \file
+
+#ifndef BLACKMISC_FALLTHROUGH_H
+#define BLACKMISC_FALLTHROUGH_H
+
+#include <QtGlobal>
+
+//! \cond
+
+// MSVC2017: [[fallthrough]] supported but only enabled with /std:c++17
+// GCC7+: [[gnu::fallthrough]] for C++11/14, [[fallthrough]] for C++17
+// XCode: Does not seem to work in current configuration
+
+#if defined(Q_CC_GNU) && Q_CC_GNU > 700
+    #if __has_cpp_attribute(fallthrough)
+        #define BLACK_FALLTHROUGH [[gnu::fallthrough]]
+    #endif
+#else
+    #define BLACK_FALLTHROUGH
+#endif
+
+//! \endcond
+
+#endif