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
We were doing this on the basis of an old comment, but there was no
test for it and I couldn't reproduce the issue with clang on Linux
at all.
Let's add a (somewhat comprehensive) test and see if it breaks
anywhere if we stop doing this.
Halves the size of gstreamer's build.ninja from 20M to 8.7M
Closes https://github.com/mesonbuild/meson/issues/1057