buildkernel avoidably building modules specified in the config

... then a lot of the code ends up being compiled twice for no good reason.

This popped up again as clang 15 emits a ton of warnings vs K&R funcs.

I don't know how this works internally, is it really a big problem to sort out?
I figured config(8) could generate a bunch of WITHOUT_ of similar, but I guess
sys/modules/Makefile will have to be patched to support it, which it
only does for some modules at the moment.

I don't have any interest in working on it, so just bringing this up
for interested.
-- 
Mateusz Guzik <mjguzik gmail.com>