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
Paths provided to us by the user or by pkg-config can be (and must be)
assumed to be usable since they might not be usable standalone.
Closes https://github.com/mesonbuild/meson/issues/3832