build: comparison tool NPE fix
build: comparison tool NPE fix

This should fix the spurious comparison tool failures.
See discussion here: https://github.com/bitcoin/bitcoin/pull/6305

The race fix was cherry-picked on top of the version we're currently using, so
it should be functionally identical otherwise.

syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin