Simplify preprocessor code that avoids the "fallthrough" C compiler warnings,...

Simplify preprocessor code that avoids the "fallthrough" C compiler warnings, assuming that "__has_attribute()" is widely supported these days. At least GCC 5.x and clang seem to have it.
Closes #1930.

Cython/Utility/ModuleSetupCode.c

@@ -516,9 +516,7 @@
   #endif
 
   #ifndef CYTHON_FALLTHROUGH
-    #if defined(__clang__) && __has_attribute(fallthrough)
-      #define CYTHON_FALLTHROUGH __attribute__((fallthrough))
-    #elif defined(__GNUC__) && defined(__attribute__)
+    #if !defined(_MSC_VER) && __has_attribute(fallthrough)
       #define CYTHON_FALLTHROUGH __attribute__((fallthrough))
     #else
       #define CYTHON_FALLTHROUGH