meson: apply our -Wno-* options also in c++ calls

meson: apply our -Wno-* options also in c++ calls

We compile some c++ code for tests. We would simply use the default options for
those. When the previous commit raised the default warning level, we started
getting warnings from c++ code. Let's add the most important options to the c++
command, so that we get a compilation without any warnings again.

I don't think it makes sense to add *all* the options that we add for c to the
c++ flags, because testing them takes quite a while, and the c++ compilations
are for small amounts of code, mostly to check that the headers have compatible
syntax.