bug#63415: 30.0.50; More Build Warnings

[Eli Zaretskii]

> From: Jacob Faibussowitsch <jacob.fai@gmail.com>
> Date: Wed, 10 May 2023 07:25:34 -0400
> 
> dispnew.c:3650:12: warning: variable 'changed_p' set but not used 
> [-Wunused-but-set-variable]
>       bool changed_p = 0, mouse_face_overwritten_p = 0;
>            ^
> 1 warning generated.

This is expected in a build --without-x.  Avoiding it is too much
trouble (will make the code harder to read), and this configuration is
rare enough to not justify that.  So I've left that alone.

> In native--compile-async:
> comp.el:4240:14: Warning: value from call to ‘cl-substitute’ is unused

Andrea, would you please take care of this warning?

I hope I've now fixed the rest, thanks.