>>598
↓これでいける？

// cxx_tunnel.h
#if defined(__cplusplus)
extern "C" {
#endif
void c_func();
#if defined(__cplusplus)
}
#endif

// cxx_tunnel.cpp
#include "cxx_tunnel.h"
#include "cxx_lib.h"
void c_func(){ cxx_func(); }

// source.c
#include "cxx_tunnel.h"
void func(){ c_func(); }