configure.ac: check for sandbox-shell's FEATURE_SH_STANDALONE
configure.ac: check for sandbox-shell's FEATURE_SH_STANDALONE

See also: https://bugs.archlinux.org/task/73998. Busybox's
FEATURE_SH_STANDALONE feature causes other busybox applets to
leak into the sandbox, where system() calls will start preferring
them over tools in $PATH. On arch, this even includes `ar`.

Let's check for this evil feature and disallow using this as a
sandbox shell.

NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix