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
using state.subdir will cause / or \ to be inserted into the target name.
Replace them with @ to future-proof it.
Signed-off-by: Marty E. Plummer <hanetzer@startmail.com>