Add ninja generator support

[Kilidsch]

When using ExternalProject, BUILD_BYPRODUCTS is needed when using the ninja generator for ninja to correctly resolve dependencies

This works well for all generators except Ninja. The Ninja build tool sees a rule listing byproduct.txt as a dependency and no rule listing it as an output. Ninja then complains that there is no way to satisfy the dependency and stops building even though there are order-only dependencies that ensure byproduct.txt will exist before its consumers need it. See discussion of this problem in Ninja Issue 760 for further details on why Ninja works this way.

From CMP0058

The above mentions custom commands, but similar is true for ExternalProject, which is used to include some dependencies

BUILD_BYPRODUCTS ...

Specifies files that will be generated by the build command but which might or might not have their modification time updated by subsequent builds. This may also be required to explicitly declare dependencies when using the Ninja generator. These ultimately get passed through as BYPRODUCTS to the build step's own underlying call to add_custom_command(), which has additional documentation.

https://cmake.org/cmake/help/latest/module/ExternalProject.html#command:externalproject_add

I have tested the changes locally on a windows machine with one release and one debug build (note that ninja on windows needs to be used from the developer prompt when using MSVC from outside of Visual Studio)