The recipe for include detection contains -x c++, and is used regardless of the actual filetype. IOW, .c and .S files are preprocessed with -x c++, which makes no sense. I'm not sure if this actually influences any behaviour (the preprocessor should behave the same regardless of filetype?), but perhaps the filetype influences the predefined macros or the handling of comments or something?

Why is this -x c++ even present? It is usually used when reading from extensionless inputs, or from pipes, but that does not happen here (perhaps this is the case when doing the preprocessing for prototype insertion, which might actually read from stdin?).