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
meson.add_dist_script, introduced in #3906, did not accept any arguments
other than script name. Since all other meson.add_*_script methods
do accept args, this makes the dist script accept them as well.