Anyone using the MacPorts version of PostgreSQL to run xTuple will run into difficulties installing the PL/v8 extension. This fix works for me.
Follow the instructions at https://github.com/xtuple/xtuple/wiki/Installing-PLv8 for Mac OS X.
Before running the "install_plv8.sh" script, edit the script and find the line that reads:
Change this line to read: