64 bit is the default for compiling the Pd core. Your problem is that you are using the old canonical 32-bit Tk Wish as provided with Pd which has not been updated with Miller's 8.5.19 build yet (as far as I remember).
You need to build Tcl, TK, & Wish yourself and then use it to build Pd.app. This is outlined in mac/README.txt
Your steps should be:
Build Pd
./autogen.sh
./configure
./make
Build Wish (and leave it around for reuse)
mac/tcltk-wish.sh --leave 8.5.19
Build Pd-0.49-1.app
mac/osx-app.sh --wish Wish-8.5.19 0.49-1
Hi, I'm doing
./autogen.sh
./configure
./make
./make app
but then I can't open Pd in Mojave as it seems to compile for an
incompatible 32 bits. Hence, I'm emailing the zipped package and opening it
in another computer with an older macOS to test it.
Can we make it compile for 64 by default? And what do I need to do to
configure it to do so?