bug#14529: gv--defsetter, advice--set-buffer-local compilation warnings

[Glenn Morris]

Stefan Monnier wrote:

>>   the function `gv--defsetter' might not be defined at runtime.
>>   (image-mode, winner, avl-tree []
>
> gv--setter is used inside a function placed on the `gv-expander'
> property of some symbols, which should never be used except by gv.el
> code (at which point gv--defsetter will be defined).

These warnings have come back following 97b7e58.
Obviously they can be declare-functioned away, but it would be nicer if
there was a general solution that did not require modifying each file
that declares a gv-setter.