CoreAnimation: warning, deleted thread with uncommitted CATransaction; set CA_DEBUG_TRANSACTIONS=1

I'm getting this warning in the log window of the debugger:

CoreAnimation: warning, deleted thread with uncommitted CATransaction; set CA_DEBUG_TRANSACTIONS=1 in environment to log backtraces.

In Xcode 5, what is the correct way to set CA_DEBUG_TRANSACTIONS=1 in the environment?

Thanks in advance to all for any info.