Bazel

Google runs the largest mono-repo known. Google uses a mono-repo because once you have solved the hurdles it makes you massively more efficient. However, you first have to use an appropriate build system. A build system that is programmed in a reasonable language (let’s call it "limited" Python) as opposed to incomprehensive text manipulation (autotools, cmake).

But more importantly, Bazel allows you to reproduce all your build - and - Bazel allows you to cache you builds. Generally, Bazel will result in massive compilation time speedups, not only for C++, but for all the stuff you do. All this is done with incredible amount of flexibility and correctness, after all it’s kind of Python.

You just have to get started.