Fix CMake multi-config support for Ninja

There is a WIP patch at https://gitlab.kitware.com/cmake/cmake/merge_requests/4086
which tries to implement multi-config support for Ninja.

Having this done in CMake would help us have a common multi-config generator on all the desktop platforms.

This is especially an improvement on Windows in comparison to MSBuild, because builds get faster.

It also gives us the ability to test and write FooConfig.cmake files which should work for multiple configurations (Debug and Release).

The feature set we expect is whatever the current Ninja generator supports + the multi config aspect that is inherent in the Xcode and MSBuild generators.