Merge pull request #3107 from RosettaCommons/roccomoretti/only_for_extras_dummy
Add dummy cc file for 'only_for_extras' build.
It appears the static build doesn't like an empty source list when an 'only_for_extras' library isn't active. Add a dummy source file which will be used for that purpose. Should fix the mac.clang.static.build.release test on the test server