Weird macOS behaviour with linking

gistfile1.txt

// a.c
#include <stdio.h>

void is_not_called(void) __attribute__((constructor)) {
  printf("This isn't called!\n");
}

// b.c
#include <stdio.h>

void is_called(void) __attribute__((constructor)) {
  printf("This is called!\n");
}

void do_something_else(void) {
  printf("Do something else!\n");
}

// main.c
#include <stdio.h>

extern void do_something_else(void);

int main(int argc, char** argv) {
  do_something_else();
  return 0;
}

// link command
clang -c a.c -o a.o && clang -c b.c -o b.o && ar qc static.a a.o b.o && clang main.c static.a -o main

// output
Neils-MacBook-Pro:weird_constructor neil$ ./main 
This is called!
Do something else!