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
Test that the host_machine is correctly detected after add_languages(),
when no langauge is initially specified in project().
In the MSYS2 MSYSTEM=MINGW32 environment (64-bit MSYS2 but with a
i686-w64-mingw32 targeted gcc as gcc) this test fails, as it
(incorrectly) tries to build retval-x86_64.S using an x86 compiler.