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).
This closes#737 issue on GitHub.
418bc208d0