setting "STANDARD_FLAGS=-std=c11" or similar will let you try out alternate compilation modes. Fixes https://github.com/libopencm3/libopencm3/issues/773
Thoughts: should this be a "sam0" family rather than samd? (Much like Atmel's own software package lumps all the cortex-m0+ devices in one family) This was enough to get a basic blinky working at least.