Let’s make it simple,

are you trying to run teensy_loader_cli on AR300M?

If yes, then it is what I have told you to do. To compile the program, you should have a cross compile toolchain rather than compile it directly on the router.

But as you have done this I hope it can work. You can just download the usb.h from libusb-compat and put it in /usr/include, but I cannot confirm it will work. The reason that there is no usb.h in the firmware is that, the router is a lightweight linux. It is used to run program but not compile program.