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
Allowing to use the new "feature" option type and allowing not to fail
on subproject if it is not necessary to fail.
By default subprojects are "required" so previous behaviour is not
changed.
Fixes#3880