bug#15880: Compute C declarations for DEFSYMs automatically.

[Paul Eggert]

Attached, finally, is an updated version of the patch which should 
address the points raised. As the ChangeLog notes, this patch uses a 
couple of ideas that improve overall performance a bit for Elisp code, 
since it means a decent C compiler can do better analysis of C code now 
that Qnil etc. are constants.  The constants are visible even to 
debuggers that don't grok macros, which I think was the main sticking 
point with the earlier approach.

Qfoo.diff
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