rsync 3.4.3 does not build on older Darwin

[debohman]

syscall.c:605:9: error: call to undeclared function 'mknodat'; ISO C99 and later do not support implicit function
      declarations [-Wimplicit-function-declaration]
  605 |                 ret = mknodat(dfd, bname, mode, dev);
      |                       ^
syscall.c:605:9: note: did you mean 'mknod'?
/usr/include/unistd.h:671:6: note: 'mknod' declared here
  671 | int      mknod(const char *, mode_t, dev_t);
      |          ^

macos 10.12.6