Ninja puts way more effort into compiling targets in parallel, and
ignores dependenceis until link time.
So we need to jump though hoops to force ninja to compile
pch.cpp before any targets which depend on the PCH.
fmt/format.h is included in the PCH, so we need to make sure fmt is
actually in the include path.
Not sure how Visual Studio + CMake manages to build without this.
Scott Mansell