To answer my own question, I believe I have what I what now - the version of ImageMagick I have just built runs on my wife’s Mac - I processed a PNG to be sure. It seems to do what I what. It seems that I had to put the path to a certain folder in the png source into PKG_CONFIG_PATH to force it to see the thing.
I wrote and sh script to compile IM with PNG libraries. It’s a result of a lot of frustration and hitting things with mallets until something sticks, so it may be a bit messy. I’ll post it here to add it to the repository of all knowledge and wisdom.