Merge pull request #229 from RosettaCommons/rhiju/allow_backtrace
This was working with no integration test changes (except the 2 sensitive ones), and I put the backtrace functions in an "ifdef __GNUC__" to only be used in 'standard' Mac/Linux builds (gcc & clang).
[I just tried a merge from master, but unfortunately several builds are broken there -- will check to see if master is restored soon.]