BOSSA: Try to fix a nondeterministic build failure.

There's a Make rule to generate .cpp from .asm but the .cpp is included
in the source. Presumably the timestamps of these two files in the tmp
dir are different when Hydra is building, but not on my system. This
causes Make to try to rebuild .cpp, but the tools needed for that are missing.
Merge pull request #4275 from ambrop72/bossa-fix

BOSSA: Try to fix a nondeterministic build failure.