WorkDir CMake 3.15+ Fix, master branch (2020.02.02.)

This is a belated update along the same lines as how all the other projects in the repository have recently been updated in !29347 (merged).

Thanks to @christos for pointing this out. (That the WorkDir project no longer works against the latest nightly...) The necessary fixes were relatively minor in the end... Also, it's worth noting that the Fortran support in the WorkDir project has been broken for a loooong time. I guess nobody wanted to develop any Fortran code against a nightly in the last 1-2 years... 😕 (It should be fixed with this.)

This is relatively urgent, as people will start complaining tomorrow morning. 😦 Also, the CI is completely blind to these updates. So the RC will have to trust me on this... 😛

Cc. @abarton, @wlampl, @emoyse, @fwinkl.