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
When we link to an external library either with find_library() without
any dirs:, or with dependency(), we should be able to run uninstalled
out of the box without having to set any environment variables or other
shenanigans.
This is especially important on macOS because only the system frameworks
directory is in the default runtime path, and all other frameworks and
libraries need to be found with RPATH or absolute path to the dylib.