bug#15880: Compute C declarations for DEFSYMs automatically.

[Stefan Monnier]

>> I'm wondering about one issue, tho: this creates a "big single file
>> dependency", so addition/removal of a DEFSYM somewhere causes
>> recompilation of all files.
> How is that different from changing variables and functions exposed to
> Lisp, and the dependency on globals.h?

Fundamentally, it's no different, but I think that in practice it's
different for one reason: the type of a Qfoo variable is always
Lisp_Object, so there's never a need to recompile because that
object changed.  So the cases where recompilation is warranted are
less frequent.


        Stefan