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

This reverts commit 6d93d57ab0.

Examples/test-suite/keyword_rename.i

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