Allow letting CMake choose the generator

Currently a generator has to be set. Clearing the field will reset it to a default (‘Unix Makefiles’).

CMake doesn’t require a generator to be specified. Without it it will simply choose one of the generators available (so it will be different on different platforms).

I suggest allowing to leave the generator field empty, and that this will result in no -G option being passed to cmake. This will allow multi-slave builds to work across different platforms. (Something I am currently trying to set up, but which fails since there’s no generator I could choose that would work on all the targeted platforms.)

(I’d be willing to implement this and send a pull request.)