Now that templates instantiations are no longer emitted for all modules that even blink at them they seem to break due to being linkonce (if compiled with any optimization level > 0), so let's give them weak linkage instead. The difference is that unreferenced linkonce symbols can be deleted, while weak symbols need to be preserved.

void main()
{
    int* e = __errno_location();
    printf("&errno = %p\n", e);
    printf("errno = %d\n", *e);
}

extern(C):
version(darwin) {
    int* __error();
    alias __error __errno_location;
} else {
    int* __errno_location();
}
int printf(char*,...);