bug#25360: File mode specification errors during building

[Glenn Morris]

Another issue that I think is related to this:
Now there are lots of free variable (and unused lexical, due to missing
defvars) warnings during bootstrapping, about autoloaded variables. The
new ldefs stuff doesn't seem to handle variables at all?

Ref eg
http://hydra.nixos.org/build/48993155/log/raw

    In toplevel form:
    files.el:750:1:Warning: Unused lexical variable `tramp-mode'

    In directory-files-recursively:
    files.el:761:10:Warning: reference to free variable `tramp-mode'

    In find-alternate-file:
    files.el:1740:17:Warning: reference to free variable `dired-directory'
    files.el:1750:15:Warning: assignment to free variable `dired-directory'

etc