Oh yeah, I should mention this here! Teensyduino 1.47 ships with a broken Makefile for the Teensy 4.0. It does not work at all. Here's how to fix it.
0. Edit ~/ardunio-1.8.9/hardware/teensy/avr/cores/teensy4/Makefile
1. Change -D__IMXRT1052__ to -D__IMXRT1062__ in DEFS.
2. Change -DF_CPU to 600000000
3. Remove -nostdlib and -nostartfiles from LDFLAGS
Yay, now it works! Use it as the basis for your next fun project.
@twylo oh! good to know. thanks!
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko