You must do this on a Linux system. Mac V8 builds (at least for Android) are broken, and the build processs generates thin archives with absolute paths. So, until the J2V8 compile is done and has produced an AAR, you'll need to stay on the same machine. The AAR, however, is portable everywhere.
These directions are the result of a lot of trial-and-error, and were written for an Arch system, but there's no reason it wouldn't work on any distro.
Note that you must be careful to reconstruct your environment if you exit your shell session!
Building V8:
mkdir v8-builder