maint: ensure bootstrap runs from dist tarball.

maint: ensure bootstrap runs from dist tarball.

* bootstrap (func_gnulib_tool_copy_file): When gnulib-tool is
not available, check that there is a copy of the file from a
previous run available before bailing out with a diagnostic.
(func_install_gnulib_non_module_files): Don't diagnose missing
files explicitly - func_gnulib_tool_copy_file takes care of it
now.
(func_require_checkout_only_file): In the diagnostic for
not being in a source-controlled tree, note that `--force' must
be used to rebootstrap.
(func_require_gnulib_files_already_present): Removed.  All the
necessary checks are correctly performed in
func_gnulib_tool_copy_file withot resorting to heuristics.
(func_require_gnulib_mk): Don't continue to try to access
gnulib-cache.m4 after discovering we're running from a dist
tarball.
(func_require_dotgitmodules_parameters): Skip if .gitmodules
file is missing and git binary is not available.
* bootstrap.conf (checkout_only_file): Set to HACKING, which is
a non-distributed file.
(libtool_cleanup_empty_dirs): Don't print spurious errors when
trying to delete gnulib-tool droppings, in case gnulib-tool is
not available on this run.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>