Summary:
In the GitHub repository this directory will be mirrored similar to
folly, such that the repository has a single top level directory
called "gloo". This allows for versioning or renaming of the
project root, without having to mangle the include paths; they will
always use the "gloo" prefix.

fbshipit-source-id: 24502e4185fc7cbe19b5249f83609e2b8118e9d7