bug#15880: Compute C declarations for DEFSYMs automatically.

[Stefan Monnier]

>> 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.

Of course, we don't want that for DEFVARs or DEFUNs.
I was thinking of doing it only for the DEFSYMs (which would obviously
need to be kept in a separate file).


        Stefan