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
As it currently stands, if for whatever reason you have two files you
want to merge with the same output name, it will fail due to multiple
targets with the same name due to the target name being autogenerated
from the output kwarg.
Signed-off-by: Marty E. Plummer <hanetzer@startmail.com>