bootstrap: Make the automatic sync more resilient. Reported by Paul Eggert in <https://lists.gnu.org/archive/html/bug-gnulib/2022-07/msg00060.html>. * top/bootstrap (bootstrap_sync): Set to true when bootstrap-funclib.sh is not present. * top/gen-bootstrap.sed: Insert a couple of comment lines. * build-aux/bootstrap: Regenerated using "make build-aux/bootstrap".