Attempt to automatically build XHPAST.

Summary: Fixes T4951. Whilst xhpast was originally intended to be an internal development tool, it is not unreasonable for ArcanistXHPASTLinter to be used externally. Currently, however, if the compiled version of xhpast is not up-to-date, we simply exit and ask the user to build XHPAST themselves. Instead, let's try to build XHPAST automatically.

Test Plan: Removed ./src/parser/xhpast/bin/xhpast and ran arc lint over a PHP file. Verified that the xhpast binary was created during this process.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4951

Differential Revision: https://secure.phabricator.com/D9076