Fix compiler warnings in clang and C++ about unknown "fallthrough" attribute.

Closes #1930.

CHANGES.rst

@@ -26,6 +26,9 @@ Bugs fixed
 * ``std::unordered_map.erase()`` was declared with an incorrect ``void`` return
   type in ``libcpp.unordered_map``.  (Github issue #1484)
 
+* Invalid use of C++ ``fallthrough`` attribute before C++11 and similar issue in clang.
+  (Github issue #1930)
+
 * Compiler crash on misnamed properties. (Github issue #1905)
 
 

Cython/Utility/ModuleSetupCode.c

@@ -507,7 +507,7 @@
 
 
 #ifndef CYTHON_FALLTHROUGH
-  #ifdef __cplusplus
+  #if defined(__cplusplus) && __cplusplus >= 201103L
     #if __has_cpp_attribute(fallthrough)
       #define CYTHON_FALLTHROUGH [[fallthrough]]
     #elif __has_cpp_attribute(clang::fallthrough)
@@ -516,7 +516,9 @@
   #endif
 
   #ifndef CYTHON_FALLTHROUGH
-    #if __has_attribute(fallthrough) || (defined(__GNUC__) && defined(__attribute__))
+    #if defined(__clang__) && __has_attribute(fallthrough)
+      #define CYTHON_FALLTHROUGH __attribute__((fallthrough))
+    #elif defined(__GNUC__) && defined(__attribute__)
       #define CYTHON_FALLTHROUGH __attribute__((fallthrough))
     #else
       #define CYTHON_FALLTHROUGH