Fix new warnings in CE2021 (#746)

Fix new warnings in CE2021 (#746)

* Initial fixes for CE2021

* Resolve conflicts with CE2020

* Update CI scripts to use CE2021

* Revert CE version for macOS CI tests

* Remove conflicting Append_Vector

* Remove unused entities in Windows

* Fix bug uncovered by CE2021

* Fix timestamps compare when mixing G.OS_Lib/A.Dirs

* Update pins for self-build

* Disable self-build, that uses CE2021

It cannot succeed until the patches in this very PR are merged, as it is
attempting to build the old code with the new compiler in the setup-alire
action.

* More fixes for relative paths in softlinks

* Remove unused entities in Windows build