Fixing detection of [[fallthrough]] attribute

cmake/tests/cxx17_fallthrough_attribute.cpp

@@ -5,17 +5,15 @@
 //  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 ////////////////////////////////////////////////////////////////////////////////
 
+#if !defined(__has_cpp_attribute)
+#  error "__has_cpp_attribute not supported, assume [[fallthrough]] is not supported"
+#else
+#  if !__has_cpp_attribute(fallthrough)
+#    error "__has_cpp_attribute(fallthrough) not supported"
+#  endif
+#endif
 
 int main()
 {
-    int n = 0, m = 0;
-    switch(n)
-    {
-        case 0:
-            ++m;
-        [[fallthrough]];
-        case 1:
-            m += 2;
-        break;
-    }
+    return 0;
 }