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
This change made `5 dependency versions` unit test fail because now once
a subproject has been configured, the fallback variable is checked to be
consistent. So it has to use new subproject because 'somesub' was
already configured by previous tests.
In the case the main project set a subproject for a dependency another
subprojects uses, that other subproject should rather use the first
subproject rather that using native dependency.
For example in gst-all we set all GStreamer modules as subprojects
and, gst-plugins-base is set after gstreamer core, and
we want gst-plugins-base to always use GStreamer core from the subproject
and not the possibly avalaible native one.