bug#15880: Compute C declarations for DEFSYMs automatically.

[Paul Eggert]

Stefan Monnier wrote:
> I mean, can the C compiler
> really presume that symbol_vals[0] won't be changed and constant-fold
> its value at link-time?

I was thinking of something that didn't have a symbol_vals table, like this:

struct Lisp_Symbol symbol_structs[N];
#define Qnil make_lisp_ptr (symbol_structs + 0, Lisp_Symbol)
#define Qt   make_lisp_ptr (symbol_structs + 1, Lisp_Symbol)
...

Qnil, Qt etc. should evaluate to compile-time constants for
any reasonably modern C compiler.