Diagnose the Link Failure

You are given (in the challenge text) two tiny sources: a.c declares and calls int helper(void); but never defines it; b.c defines static int helper(void) { return 1; }. Implement int link_outcome(void) returning the code for what linking a.c and b.c produces: 0 = links and helper returns 1, 1 = undefined reference at link time, 2 = multiple definition.