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
because down the import dependency chain of copy.py the 'tarfile' module
was trying to import the 'copy' standard library module but was finding
the copy.py file first because it was in the current directory.
Closes https://github.com/mesonbuild/meson/issues/1359