It is quite common with our customers that they want to be able to control this kind of stuff at the installation level. They either have one very large project or multiple projects they want to follow the same standards.
I don’t doubt that at all; I’m aware of the xlc.cfg, vacpp.cfg, and similar files and I don’t doubt that customers modify them. But I think that is a pattern that should not be encouraged in upstream Clang as it creates difficulties for project build systems; they can’t tailor their configuration to a given compiler particularly well if the behavior of the compiler changes on a site-by-site basis. I have no concerns with IBM implementing such behavior in its Clang distributions of course.