get_filename_component ABSOLUTE does not return drive letter on windows
```
>cmake --version
cmake version 3.15.0
>type 1.cmake
get_filename_component(name / ABSOLUTE)
message(${name})
>cmake -P 1.cmake
/
```
`REALPATH` returns correct path `C:\`. I'm getting the same result in Windows Server 2019, 19.03 and docker.
As a result protocol buffer compiler is not working correctly, changing `ABSOLUTE` to `REALPATH` in `FindProtobuf.cmake` helps, but it is not the best solution.
issue