disable [[fallthrough]] until C++17

Fixes #215, reported by Thibaud Michaud.

Also related to GCC bug 79301.

* spot/misc/common.hh: Here.

spot/misc/common.hh

 // -*- coding: utf-8 -*-
-// Copyright (C) 2013, 2014, 2015, 2016 Laboratoire de Recherche et
+// Copyright (C) 2013, 2014, 2015, 2016, 2017 Laboratoire de Recherche et
 // Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -125,15 +125,18 @@
 //   auto func(int param) SPOT_RETURN(implem_.func(param));
 #define SPOT_RETURN(code) -> decltype(code) { return code; }
 
-// We hope compilers that implement -Wimplicit-fallthrough
-// also support __has_cpp_attribute and some form of [[fallthrough]].
+// We hope compilers that implement -Wimplicit-fallthrough also
+// support __has_cpp_attribute and some form of [[fallthrough]].  Do
+// not use [[fallthough]] if the code is compiled in a pre-C++17
+// standard since clang's -Wpedantic would complain that we are using
+// a feature from the future.
 #ifdef __has_cpp_attribute
-#  if __has_cpp_attribute(fallthrough)
+#  if __has_cpp_attribute(fallthrough) && __cplusplus > 201402L
 #    define SPOT_FALLTHROUGH [[fallthrough]]
 #  elif __has_cpp_attribute(clang::fallthrough)
 #    define SPOT_FALLTHROUGH [[clang::fallthrough]]
-#  elif __has_cpp_attribute(gcc::fallthrough)
-#    define SPOT_FALLTHROUGH [[gcc::fallthrough]]
+#  elif __has_cpp_attribute(gnu::fallthrough)
+#    define SPOT_FALLTHROUGH [[gnu::fallthrough]]
 #  endif
 #endif
 #ifndef SPOT_FALLTHROUGH