[Too]

Because you included a third-party project into your build which is not compatible with your build system and you don't want to rewrite all their ninjafiles into makefiles to match.

Someone should really formalize a standard for declaring dependencies which build systems can share between each other.

[klodolph]

A “standard for declaring dependencies” is, itself, a build system without the execution engine. The execution engine is not actually the hard part about making a build system. So, what you’re asking for is really just a standardized build system.