Some non-Linux systems implement pivot_root(2), even if they don't document that. An example is MacOS:
$ grepc pivot_root / 2>/dev/null .../sys/sysproto.h:3012: int pivot_root(struct proc *, struct pivot_root_args *, int *);
Since the prototype of the syscall differs from that of Linux, we can't use that syscall. Let's make sure the test only detects pivot_root(2) under Linux. Also, rename the feature macro to make clear that it's only about Linux's pivot_root(2).