Re. commit 517c704a6360356e62752f581c71b38a479e2f59
NS has it’s own built‐in image support, so therefore doesn’t define
HAVE_PNG, etc.
What was the actual compiler warning?
I don't remember the exact wording, but it was that the static function is unused. That makes sense, because macOS doesn't call x_query_frame_background_color unless one of the graphic libraries is also available.