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
The system tool is always the wrong thing to use and cause hard to debug
issues when trying to link system libraries with cross built binaries.
The ExternalDependency base class already had a method to deal with
this, used by PkgConfigDependency and QtBaseDependency, so it should
make things more consistent.