Don't use C++ features when building test code as C.

This fixes the build after a6efdb7 which added a method to a test used in both
C++ and C test cases.

Examples/test-suite/keyword_rename.i

@@ -35,9 +35,11 @@ KW(chan, fallthrough)
 KW(end, function)
 KW(nil,local)
 
+#ifdef __cplusplus
 struct Keywords {
   void selfkw(int self) {}
 };
+#endif /* __cplusplus */
 
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