Fix previous commit to compile with older compilers

Not all "C++11" compilers will also support C++17.

Examples/test-suite/cpp11_static_assert.i

@@ -8,14 +8,24 @@ static_assert(sizeof(int) >= 2, "What?  int size is invalid!");
 
 namespace dummy {
 // C++17 allows the message to be omitted, so check that works too.
+// But only show the C++17 version to SWIG, as the compiler may
+// lack C++17 support.
+#ifdef SWIG
 static_assert(sizeof(int) >= sizeof(short));
+#else
+static_assert(sizeof(int) >= sizeof(short), "blah");
+#endif
 }
 
 template <typename T>
 struct Check1 {
   static_assert(sizeof(int) <= sizeof(T), "not big enough");
   Check1() {
+#ifdef SWIG
       static_assert(true);
+#else
+      static_assert(true, "true");
+#endif
   }
 };