How do you deal with things like system headers then? Force everyone to check in their OS (bye BSDs)? Assume everyone runs the same OS?

How do you deal with compiler provided internals, where someone might be using clang which has behavior X, but gcc / icc / msvc doesn't?

These problems are more prominent in C/C++, but are everywhere... Not easy...

Google can probably check in everything for themselves... Probably not so easy for other companies? Maybe? Can Google solve this without sinking too much resources into it?