Add support for the Perforce VCS client via the "p4" tool

[Perforce](http://www.perforce.com/product/perforce) is a version control system.

This patch uses the command-line "p4" tool to fetch dependencies stored under Perforce.  It's a little odder than the other VCS clients because of Perforce's notion of a "workspace" that's tracked by the P4 server.