meson: Ignore unused variables when assertions are disabled

meson: Ignore unused variables when assertions are disabled

Fixes: 46b099e3
("meson: Ignore unused variables in release builds")

46b099e3 has some issues:
- it doesn't enable unused variables warning on release builds
with assertions enabled;
- it doesn't disable unused variables warning on debug builds
with assertions disabled;
- it doesn't disable unused variables warning when building
with MSVC and assertions are disabled regardless of buildtype,
see #8147. 3/4 regressions reported there have this limitation
alone as root cause.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21154>