>Rule #4: Use only built in #ifdef compiler flags, do not invent your own
Actually you can separate the source code into src/linux src/w32 src/mac -- but that is just my preference.
>Rule #9: Require all programmers to compile on all platforms
No no no -- build has to take no more than a make command. Anything else is just domed to failure.
IMO the author suggests to develop in a single source tree which compiles on every platform for very good reasons, in particular to minimize duplication and being able to update a single method in a single file to work on all platforms. Your suggested multi-directory approach requires to duplicate most os-specific methods and classes. Depending on the situation one or the other approach might be better, but I think the author has a point in many of them.
> No no no -- build has to take no more than a make command. Anything else is just domed to failure.
Building with just a "make" should be fine on Linux, but for anyone who downloads the source code to a Windows/VS or OSX/XCode machine, it's great being able to just open the project in the standard editor and compile it.
Small matter, easily detected and fixed in the first build, but would be nice to automate somehow.
In any case, I used CMake in one shop I worked in and it was nice to have the build files all managed in one place. We were able to manage builds for Windows, Linux, Mac, and Solaris.