gpc.c를 컴파일하다가 발생한 에러
[해법]
The extern "C" construct is a C++ specific thing, it can’t be used in C. And the compiler treats your source file as a C source file since it has the extension .c.
[참고문헌]
https://stackoverflow.com/questions/16192872/extern-c-error-2040-expected-an-identifier
https://shoon950.tistory.com/entry/함수-오버로딩-extern-C-cplusplus-매크로
https://dojang.io/mod/page/view.php?id=802
[교훈]
확장자 .c와 .cpp의 차이를 기억하자.