enable google perftools, e.g. heap profiler, cpu profiler, to work

After we changed all the exit()'s to _exit()'s, this broke the way google perf tools work, since they register an exit handler to write out the profiler data. This would be easy to fix; just add conditional compilation to call ::exit instead of ::_exit when using --use-cpu-profiler SCons flag. Unfortunately, it means that we can't easily ask users to turn on heap profiling just via environment variables, like we could with 2.0.7.

I'm not sure if calling ProfilerStop() explicitly writes out the profile right then or if it waits until process exit.