Complete Makefile to build shared C++ library

[machinekoder]

The Makefile installs C++ headers, however, does not build any library. To make use of machinetalk-protobuf it would make sense to move building the shared machinetalk-protobuf library to this repository.

[machinekoder]

Related to #24

[machinekoder]

I suggest adding the build of the library to a cpp sub-directory to keep consistency with the js and python libs.