Here is my progress so far.
The json-c package that comes with MacPorts was no good. I just followed the instructions here https://github.com/json-c/json-c/blob/master/README and that issues was resolved.
I have Zend Server. when I ran the ext/install it used the wrong phpize. I just get rid the one that ships with OS X that is in /usr/bin.
Everything seemed to build OK, but I'm getting a "wrong architecture" error. when trying to use the complied extension.
I'll try some more and update when I make progress.
I understand this is early days, I'm just very excited about the idea and wanted to get my hands dirty.