[th/meson-build-type-fix] build/meson: fix build to always set "-W" compiler flags

We must set these compiler flags independent as to whether this is a release build or a debug build.

In most cases, we don't differentiate between release and debug build anyway. Granted, we have "-D more_asserts=100" and set "-O" CFLAGS, but that is more granular and not a simple "buildtype".

In particular, these compiler flags apply to all kinds of builds.

This is important, because otherwise we get build failures, because also in release build we want to build with -Werror and -Wall.