Extensibility for more complex build steps

I really like the idea of this, I wish build systems were less intrusive and this seems to do something toward that. I feel however it would be useful to have more extensibility to run tools such as protoc (Protobuf compiler) to generate code, before then compiling it, as currently I can't see any way of doing this. If you really wanted to stick with the whole everything is C aspect, you could maybe even use TCC or such to achieve this, but I'm not sure it's really necessary to go to those lengths.