mkdir build

Car.cpp

#include <stdio.h>
#include "SomeFunctions.h"
#include "Car.h"

void Car::print() {
  printf("Car::print()\n");
  test_function();
}

Car.h

#ifndef CAR_H
#define CAR_H

class Car {
 public:
  void print();
};

#endif

CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
project(PluginTest)

add_library(Transportation STATIC Car.cpp SomeFunctions.c)
add_library(Plugin MODULE Plugin.cpp)
target_link_libraries(Plugin Transportation)

main.cpp

#include <stdio.h>
#include <stdlib.h>

int main() {

  printf("Test with Plugin.\n");
  
  return 0;
}

Plugin.cpp

#include <stdio.h>
#include "Plugin.h"
#include "Car.h"

void plugin_init() {
  printf("plugin_init().\n");
  Car car;
  car.print();
}

void plugin_update() {
  printf("plugin_update().\n");
}

Plugin.h

#ifndef PLUGIN_H
#define PLUGIN_H

#if defined(__cplusplus)
extern "C" {
#endif

  void plugin_init();
  void plugin_update();
  
#if defined(__cplusplus)
}
#endif
  
#endif

SomeFunctions.c

#include <stdio.h>

void test_function() {
  printf("test function.\n");
}

SomeFunctions.h

#ifndef SOME_FUNCTIONS_H
#define SOME_FUNCTIONS_H

void test_function();

#endif