Fix C++20 compatibility in testcases

Examples/test-suite/template_construct.i

@@ -7,7 +7,13 @@ template<class T>
 class Foo {
     T y;
 public:
+#ifdef SWIG
     Foo<T>(T x) : y(x) { }
+#else
+    // Modern compilers reject this, so feed the compiler the corrected
+    // version.
+    Foo(T x) : y(x) { }
+#endif
 };
 
 %}