Don't assume itstool, msgfmt et al. are just magically on the path.
Normally for commands being processed in build.ninja we'd look up the
program in order to run it. Offer the same guarantee for programs being
passed through an awkward script wrapper.
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