bug#55846: make bootstrap -j2 fails on macOS

[Mattias Engdegård]

nextstep/Makefile.in has the rule

${ns_check_file} ${ns_appdir}: ${srcdir}/${ns_appsrc} ${ns_appsrc}
        rm -rf ${ns_appdir}
        ${MKDIR_P} ${ns_appdir}
        ( cd ${srcdir}/${ns_appsrc} ; tar cfh - . ) | \
          ( cd ${ns_appdir} ; umask 022; tar xf - )
        [ "`cd ${srcdir} && /bin/pwd`" = "`/bin/pwd`" ] || \
          ( cd ${ns_appsrc} ; tar cfh - . ) | \
            ( cd ${ns_appdir} ; umask 022; tar xf - )
        touch ${ns_appdir}

but because of its dual targets, ns_check_file and ns_appdir, it is unsafe to 
use in parallel builds and seems in fact often to be invoked twice at the same 
time, which leads to general confusion and eventual build failure.