Currently the unit tests `test_util_spawn_background_`* assume that they are run from the Tor build directory (so as to find `test-child`). This is not the case when running `make distcheck`, so the test will fail. The path to `test-child` can be found through `dirname(argv[0])/../test-child`, but unit tests don't have access to `argv[0]`. Therefore one solution is to use autoconf to find the Tor build directory and base the path on this. Originally reported by Sebastian Hahn.