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
Since Vala require 'glib-2.0' and 'gobject-2.0' dependencies, it's
better to fail at 'valac' step with meaningful error.
Add missing 'gobject-2.0' dependency on the mixed source test case.