This check puts all of our code in a C++ anonymous namespace, which is
effectively making all functions `static`. This allows the compiler to
determine that a function is unused, so we can delete it.
This allows Tox to contain additional data on top of Messenger, making
Messenger not necessarily the most top-level object. E.g. groups are
built on Messenger and currently awkwardly void-pointered into it to
pretend there is no cyclic dependency.
