Summary:
In some cases (notably, homebrew) an installer may not control where arcanist/ and libphutil/ live and may not be able to control 'include_path'.
Allow libphutil/ to be symlinked into arcanist/externals/includes/ if all else fails.
Test Plan:
- Moved `libphutil` to `libphutilx`. Ran "arc" and got a failure.
- Symlinked it into externals/includes/, ran `arc`, got success.
- Moved it back to `libphutil`, ran `arc`, success.
Reviewers: vrana, btrahan
Reviewed By: vrana
CC: aran, tfmeusburger
Differential Revision: https://secure.phabricator.com/D3243
