Refactor external project arguments to use a list instead

[dacap]

Not sure if CMAKE_ARGS_FOR_EXTERNAL_PROJECT is a standard name, but if it was created by us, I'd replace the CMAKE_ prefix with LAF_.

[martincapello]

You are right, it is not standard. I've named this way because they are arguments for the cmake command executed by the ExternalProject_Add function. I'll change it.