When I worked at Google I built a Blaze extension to be able to build Android apps. It worked really well, though I'm not sure how well it was maintained after I left in 2010. Internally at Google, Blaze was extremely customizable, and I hope Bazel too, so one can easily add support for building iOS apps etc.

EDIT #1: I see support for building Objective-C apps is already present in Bazel. EDIT #2: Bazel uses Skylark, a Python-like language, which could be used to implement all sorts of extensions, including the one I was referring to.

There's an extension language in bazel named Skylark, which will be familiar to you if you wrote build_defs internally: http://bazel.io/docs/skylark/concepts.html

The Chromium tool chain is pretty insane. ninja, fetch, etc... There's really no excuse for this since Google has such a strong (and now open source) build system.