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
With this running `meson compile` inside the builddir actually works,
and you don't have to do `meson compile -C .`
Clearly this was the intent since the option already has `default='.'`