// mine.h
#ifndef MINE_H
  #ifdef MINE_C
    #define EXTERN 
  #else
    #define EXTERN extern
  #endif
  EXTERN  int my_int;
                // the below is not needed and ...
  #undef EXTERN // could be the problem
#endif

1) with the exception of the comment I inserted in your code, this is fairly standard for some of us, others scream whenever they see it.

2) the #ifndef MINE_H has no function if not followed by #define MINE_H. Many swear by it but I consider it a means of "allowing screwups"

3) I dislike the use of EXTERN, it is confusing, a name like PQW or whatever makes it clear that if 'extern' appear, in either upper or lower case, this is an external. This is a dislike, nor a "problem".

4) in the same line I do not like using "MINE_C" some will assume this is an automatic. This is a dislike, nor a "problem".

create a simple case add post the two "C" and the "H" module.

Erik