D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082
Otherwise we might end up with wrapper holders in the Build object and
pickling will then fail, defeating the purpose of the holder objects.
Closes https://github.com/mesonbuild/meson/issues/2211