Try detecting clang::fallthrough using __has_cpp_attribute

and use it to silence one of the warnings in #96.

format.h

@@ -76,6 +76,12 @@
 # define FMT_HAS_BUILTIN(x) 0
 #endif
 
+#ifdef __has_cpp_attribute
+# define FMT_HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
+#else
+# define FMT_HAS_CPP_ATTRIBUTE(x) 0
+#endif
+
 #ifndef FMT_USE_VARIADIC_TEMPLATES
 // Variadic templates are available in GCC since version 4.4
 // (http://gcc.gnu.org/projects/cxx0x.html) and in Visual C++
@@ -109,6 +115,12 @@
 # define FMT_NOEXCEPT(expr)
 #endif
 
+#if FMT_HAS_CPP_ATTRIBUTE(clang::fallthrough)
+# define FMT_FALLTHROUGH [[clang::fallthrough]]
+#else
+# define FMT_FALLTHROUGH
+#endif
+
 // A macro to disallow the copy constructor and operator= functions
 // This should be used in the private: declarations for a class
 #define FMT_DISALLOW_COPY_AND_ASSIGN(TypeName) \
@@ -864,7 +876,7 @@ class ArgVisitor {
     switch (arg.type) {
     default:
       assert(false);
-      // Fall through.
+      FMT_FALLTHROUGH;
     case Arg::INT:
       return FMT_DISPATCH(visit_int(arg.int_value));
     case Arg::UINT: