build.sh: Allow calling from other directory

No, you could take out a part like the `BASH_SOURCE` thing (that's basically to be agnostic to execution via `./script.sh` vs inclusion via `source ./script.sh`), but not the `cd && pwd` part.

In the end, this became my reliable boilerplate code for "path to my location". See http://stackoverflow.com/a/246128/1034080 for (possibly) all permutations on and reasons behind this.