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
Points to the `mesonintrospect.py` script corresponding to the
currently-running version of Meson.
Includes a test for all three methods of running scripts/commands.
Closes https://github.com/mesonbuild/meson/issues/1385