There was already a minimum version specified in the actual CMakeLists
file in the C++ directory, but since the parent file does not have it
you get a warning if you build in the idiomatic cmake fashion:

    mkdir build
    cd build
    cmake .. # <---- warning here because no minimum version specified

I used 3.1 as min version because that is the same as specified in child
file.

The idea behind the directory organization was that we might have official implementations for other languages in other top-level directories, as siblings to the c++ directory. It seems implausible that we'd use CMake as a meta-build-system over all of these.

Another reason for this change is that the release tarball actually contains only the c++ subdirectory. We probably want to include cmake files in the release.