[Limiting to gnulib]
+ - Don't use the flags -std=c99 or -std=gnu99.This is not a realistic option as far as I can tell. As the user, I never specified -std=gnu99. coreutils and/or gnulib and/or gcc "helpfully" inserted it somewhere, somehow. I have no practical way (that I can see) to omit it. Thanks to all for the follow-up on my obscure issue ...
