Checking for CLONE_NEWNS is only needed for using tha Linux specific clone(2),
otherwise we can use fork(2). Using clone on the Hurd needs some work, only
support LINUX for now. See
https://lists.gnu.org/archive/html/guix-devel/2020-03/msg00190.html
* nix/libstore/build.cc (CHROOT_ENABLED): Break into CHROOT_ENABLED
and CLONE_ENABLED.
(DerivationGoal::startBuilder): Replace CHROOT_ENABLED with __linux__.
(DerivationGoal::runChild): Only define pivot_root() if SYS_pivot_root is
defined.
Co-authored-by: Jan Nieuwenhuizen <janneke@gnu.org>
9c3b28b911