Move XHPAST build script to PHP

Summary:
Currently, building XHPAST from within PHP involves running PhutilXHPASTBinary::build(), which executes the ./scripts/build_xhpast.sh script, which then calls make. Instead, we can bypass ./scripts/build_xhpast.sh and call make directly from PhutilXHPASTBinary::build(). This feels a lot cleaner.

I have retained the ./scripts/build_xhpast.sh script, although it is really a PHP file now. I was considering renaming it to build_xhpast.php, but decided to leave it as is for backwards compatibility.

Test Plan:

> ./scripts/build_xhpast.sh
Build successful!

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11629