use a macro to set GUIDIR
This makes it easier to point the gui dir at a local copy of the repo for easier development. You can use this to make it easier to test out changes to the GUI in purr-data/pd/nw.
Setting GUIDIR should even work if you haven't installed Purr Data to the system. Just make sure the nw.js binary has been downloaded and expanded to purr-data/pd/nw/nw. (This happens automatically when you build.)
I thought about using a startup flag to make it possible to set this on the command line. I don't personally gain anything from that in my own dev process, but if other people want that it shouldn't be hard to implement later.