Summary:
Depends on D21063. Ref T13492. Currently, XHPAST defines a version in both PHP code and C code, and they must be kept in sync.
Switch to a single definition in PHP, then carry it through the build pipeline into C.
Test Plan: Did a clean rebuild of XHPAST, saw a version number carried in from PHP. Ran "xhpast --version".
Maniphest Tasks: T13492
Differential Revision: https://secure.phabricator.com/D21064
