Somewhat replaces some earlier work done by hg/lg, but much more complete, so we kept it as is, because it's bringing in even more parts after this. Reviewed-by: Karl Palsson <karlp@tweak.net.au>
setting "STANDARD_FLAGS=-std=c11" or similar will let you try out alternate compilation modes. Fixes https://github.com/libopencm3/libopencm3/issues/773