Add support for >=msgpack-5.0.0

[mkszuba]

Since version 5.0.0 the CMake package installed by msgpack-c C++ library has been called "msgpackc-cxx". Seeing as mmtf-cpp build scripts look for that package it would be good for them to account for this change, even though as far as I can see all that it actually results in is a warning produced by cmake.

[gtauriello]

Given that MMTF support has been discontinued by the PDB (see announcement here), I won't be doing further updates on this package. I apologize for any inconvenience caused by this.