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
Of course D compilers have different flags to set some important
D-specific settings. This adds a simple method to change these flags in
a compiler-agnostic way in Meson.
This replaces the previous `unittest_args` method with a more generic
variant.