Building Foundation on Linux with incremental builds?

Does anyone know how to compile Foundation (with Ninja) on Ubuntu Linux
16.04 with Incremental builds enabled? It currently takes over 4 min on my
8 core, 16 GB machine because it rebuilds the project from the beginning
each time. Any way to make the development to test cycle faster would be

  I have inspected the file and it seems to be fine, however
running `ninja` or `ninja test` will rebuild the entire project. In other
words, when I make one line of code to URLSession.swift, it ends up
rebuilding NSCharacterSet.swift and NSPersonNameComponents.swift, etc. Over
100 source files before building URLSession.swift and finally linking