Build errors

Trying to build on ubuntu 20.04 LTS and at stage 18 of the build I get the following error. 

```
CMake Error at cmake/StdFilesystem.cmake:22 (message):
  Toolchain doesn't support std::filesystem with nor without -lstdc++fs
Call Stack (most recent call first):
  cmake/StandardSettings.cmake:57 (include)
  CMakeLists.txt:70 (include)
```