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
On Windows, we can build with both 32-bit and 64-bit compilers, but the
Python is either 32-bit or 64-bit. Check the architecture of the found
Python libraries and don't use them if they don't match our
build_machine.
Also skip the tests if the Python 3 dependency is not found.