bstracing: Include time spent on building libraries

[jchavarri]

Currently, bstracing only exports time from "top level" application compilation time, but it'd be great if it also included —maybe conditionally through a command line flag— the time spent in libraries compilation, in order to have a more complete profiling picture.

[bobzhang]

For the temporary work around, you can run bstracing in your dependencies independently

[bobzhang]

we will add bstracing -C dir #4131