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
A custom_target, if install is set to true, will always be built by
default even if build_by_default is explicitly set to false.
Ensure that this does not happen if it's set explicitly. To keep
backward compatibility, if build_by_default is not set explicitly and
install is true, set build_by_default to true.
Fixes#4107