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 (closed). 3/4 regressions reported there have this limitation alone as root cause.
/cc @jenatali @zmike @dbaker @alyssa @anholt @lygstate @eric @kusma