Meson will implicit rpaths when *.so/*.dll/etc. files are injected onto
the link line from pkg-config and (now) cmake dependencies.
Extend the "prebuilt shared" tests to test that these are added.
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