suppress set but not used warnings for kernel builds

[rew]

Summary

Turn off set but not used warnings for kernel builds with clang13.

These warnings can be turned back on by setting the following in src.conf:

WITH_SET_BUT_NOTUSED_KERNEL_WARNINGS=

[imp]

this breaks boostrapping

sys/conf/kmod.mk

119 ↗

(On Diff #104593)

use ${NO_WUNUSED_BUT_SET_VARIABLE} instead.

[rew]

feedback from imp: use ${NO_WUNUSED_BUT_SET_VARIABLE}

[imp]

I have ~65 changes in my queue to eliminate all the trivial errors for the arm kernel build.
I plan on pushing them shortly, and there may be some overlap with your work, but I've finished universe with them...

[mjg]

Modules get built even if particular code is built-in into the kernel, artificially raising the clang spam.

The fix would be to stop building said modules to begin with.

[rew]

Taking this review in a different direction..

Build kernel with -Wno-unused-but-set-variable by default.

[imp]

sys/conf/kern.mk

46

I'd be tempted to merge just this one line to stable/13 and not make it an option there.

[rew]

sys/conf/kern.mk

46

agreed.

...wonder if it'd be better to drop the option now?

[mjg]

just commit and mess with it later