This flag isn't being used in any of our deployments. I sure hope not,
anyway, because if you turn it off, things are mega-busted; I can't even
run a sandbox without crashing pretty early on.
This breaks the OSS build's dependency on xhp. We're still depending on
it internally in a Facebook-specific extension (exposing the XHP
preprocessor to PHP code as ##xhp_preprocess_code()##). There might be
some way to replicate this functionality using HPHP's native XHP parser,
but that seems low-pri.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.
mwilliams