Right, it seems that Teensyduino does not have C99 mode enabled, like the regular Arduino core has since 1.6.6. I've contacted Paul (Teensy maintainer) about this, I'll keep you posted. For now, you can probably fix this by finding the Teensy platform.txt, looking for a line starting with compiler.c.flags and adding the -std=gnu11 op;tion (this enables C11 mode with GNU extensions, which includes C99).
A second problem is that the printf support I used turns out to be AVR-specific. You can either disable it in config.h, or try this branch https://github.com/matthijskooijman/arduino-lmic/tree/non-avr-printf (I'll merge this to master once someone confirms printf works on Teensy or some other non-avr platform with it).