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
In version 3.6.0, clang added -Wignored-optimization-argument. Without
setting this flag to -Werror, "ignored optimization arguments" such as
(for example) -fpeel-loops, are accepted but warned about, leading to
noisy builds if meson thinks the flag is supported.
See also #755