Normally, you can build xhpast by running make, which will create a binary in this directory:
xhpast/ $ make
You can run make install to copy that binary to the right place in libphutil so that libphutil, Phabricator, Diviner, etc., can use xhpast.
If you are developing xhpast, you need to make parser scanner xhpast instead to regenerate the parser and scanner:
xhpast/ $ make parser scanner xhpast
We ship the generated parser and scanner and do not rebuild them by default to prevent users from needing to install flex and bison and fiddle around with flex versions.