[msgpack-cxx] Please provide a pkg-config file

[stephanlachnit]

It would be nice if msgpack-cxx could be found via pkg-config in addition to CMake. This is helpful for other build systems and platforms where CMake cannot be used.

I started adding a pkg-config file here: stephanlachnit@fd5ae77

However, this does not export the compile options / definitions. I am not quite sure how to extract those from CMake.