admin/merge-gnulib now warns if module lists change

admin/merge-gnulib now warns if module lists change

From suggestions by Eli Zaretski and Stefan Kangas in:
https://lists.gnu.org/r/emacs-devel/2025-01/msg00036.html
https://lists.gnu.org/r/emacs-devel/2025-01/msg00037.html
* admin/merge-gnulib: For style consistency,
prefer ‘[ EXPR ]’ to ‘test EXPR’ and prefer -e to -f.
Fix bug that forgot to clean lib directory if it’s ./lib.
Check for lib/gnulib.mk.in, which we used directly, not ‘configure’.
(LC_ALL): Set to C.
(autogen): New function, to make it clearer to user when
commands are operating in some other directory.
(get_module_list): New function.
(old_modules, new_modules): New vars.
If they differ, issue a warning.

(cherry picked from commit f65520cad46273967da2343feeb2201363c190e0)