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
`-Dtest_harmless_but_useless_link_arg` won't actually do anything
without anything to preprocess, but least it is valid for GCC and MSVC,
and won't be caused by anything else.
When passing more than one -Dc_args it should override the value
instead of appending. This is how all other options works.
Value should be split on spaces using shlex just like it does with
CFLAGS environment variable.
Fixes#3473.