Set C++ standard version automatically

Instead of using

```
cmd(f'{preconfigure}cmake -S MaGe/source -B MaGe/build -DCMAKE_CXX_STANDARD=14 -DCMAKE_INSTALL_PREFIX={install_path}')
```

we use get the C++ standard version ROOT was built with, e.g. via `root-config --cflags | sed 's/.*std=c++\([0-9]\+\).*/\1/'`