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
These paths are now generated similar to
NinjaBackend.generate_single_compile where IncludeDirs create includes
of both the build directory path and the source directory path.
This also fixes a bug with include_directories, where the path string
supplied to the IncludeDirs initializer was used for the search path,
instead of the actual location to which it referred. Often, this was a
'.', and not a really useful path.