should any build system legitimately change any tracked files?

  just finished teaching a couple git courses and, after class, a
student came up and described a rather weird problem -- in short:

  1) before build, "git diff" shows nothing
  2) do the standard build
  3) suddenly, "git diff" shows some changes

that's all the info i was given, but it *seems* clear that the build process itself was making changes to one or more tracked files.

  technically, i guess one can design a build system to do pretty
much anything, but is it fair to say that this is a really poor design
decision? admittedly, this isn't specifically a git question, but i'm
open to opinions on something that strikes me as a bad idea.

rday