Give setup.py the ability to produce a debug build of ConsensusCore.

You can get a debug build (all assertions, all debug symbols, etc.)
via

    $ python setup.py install -g

or

    $ python setup.py install --debug

[Bug 22178]
prop:1.4

[git-p4: depot-paths = "//depot/software/common/ConsensusCore/": change = 118151]