Most other related formulae in Homebrew/core are suffixed with `-cpp`
rather than `-cxx`. Also, the upstream branch this formula is built on
is called `cpp_master`, despite the actual artefact being called
`msgpack-cxx`.
This makes it likely that some users are going to try to do `brew
install msgpack-cpp` and end up confused by the resulting error. Let's
try to avoid that by adding this alias.