* bootstrap (symlink_to_gnulib): New function.

* bootstrap (symlink_to_gnulib): New function.
(cp_mark_as_generated): Use it, to prefer symlinks-to-gnulib
to copies-of-gnulib.
(cp_mark_as_generated, slurp, gnulib_files):
Avoid making a copy if it's the same as the old version.
(gnulib_files): Add support for this variable (used by Bison).