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
The documentation doesn't require it and the interpreter code works around the
possibility of it being None. The ninja backend code however fails with
File "/home/whot/code/meson/mesonbuild/backend/ninjabackend.py", line 796, in generate_data_install
dstabs = os.path.join(subdir or None, plain_f)
File "/usr/lib64/python3.6/posixpath.py", line 78, in join
a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType
If install_dir is missing, default to datadir/projectname