Please change ninja process exit code for 'no work to do'

[LostTime76]

It would be beneficial if the exit code of the ninja process for 'no work to do' = 0 and 'build success' = 0 are differentiated with different values. The reason is because there is a use case where one may want to determine if anything was actually built and run something else based on this distinction. For my use case, I am just using ninja to compile. nothing else. If something was actually compiled, I need to relink, but if nothing was built, I don't need to relink. Before I relink I have to generate additional files each time. In the non relink case I want to avoid generating those files.

I think this could be done several ways.

1. Either you just change the return code in the 'no work to do case' to something unique and other than 0
2. Provide a command line argument for users that want to do what I am doing so they can change the code themselves:
   --no_work_to_do_exit_code=2. This would keep backwards compatibility for existing tools.

For now, I have checked out the code myself and it was easy to build it. This is the change I made and it works for what we need.

[scivision]

In my opinion, this should be a command line option as you suggest. Thinking like curl etc.