Re: built-in variable `font-use-system-font' not bound

[Jason Rumney <jasonr@gnu.org>]

Stefan Monnier wrote:
> Regardless of how things currently are (we currently have a big mess in
> this respect), we should follow the following convention:
> - if it's a feature specific to X11, W32, NS, ... then use a name that
>   starts with "x-", "w32-", "ns-", ...
> - if it's a feature that makes sense everywhere, then don't use such
>   a prefix (even if it's only implemented for some of the backends).
>   

If we follow that convention, I think we need another prefix for 
functions/variables that are common across window systems that can be 
used in the test to avoid warnings/errors in cus-start.el for tty only 
builds.   Currently we usually use x- for these, since historically most 
such features started on X and were ported to other window systems later.