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Setting -fno-rewrite-rules means we still test out the vectoriser, but 
don't spend ages optimising the generated core code.

This also made the "run_command $MAKE" tests fail, as they were also
sending this to stderr.

The arity patch improves debugger output slightly

The test modifies the user database, and you might get
   "WARNING: there are broken packages" 
for no fault of your own, which messes up validate