bug#15880: Compute C declarations for DEFSYMs automatically.

[Paul Eggert]

Stefan Monnier wrote:
> this creates a "big single file
> dependency", so addition/removal of a DEFSYM somewhere causes
> recompilation of all files.  Would it be possible to trick the deps/*
> rules so they pretend this file is not a dependency?

Possibly, but that would run into the opposite problem.
If you add or remove DEFVAR_LISP somewhere so that
the layout of struct emacs_globals changes, then
omitting the dependency and compiling some files
but not others would make Emacs misbehave badly.