Most systems do not have pax installed by default, or it is not available
at all. However, since we are relying on the -s flag for member name
substitution, we can't just use a tar command that works everywhere.
Instead, support the environment variable PAXREAD to specify the command
to use in place of `pax -r`. Since bsdtar from libarchive supports -s,
we can set PAXREAD='bsdtar -xf -'.
Since we are now using Lua to generate ninja files, use of rc in build scripts
seems unnecessary and adds an additional bootstrap dependency. None of them are
too fancy, so just port to POSIX sh instead.
