extra build dependencies

[incanus]

I wanted to build all the platforms, but I'm on a Mac so I just used Docker to spin up a debian:unstable image to simplify things since you call it out specifically.

The docs say:

On Debian unstable, you can get everything from the main archive:

        # apt install lola gcc-avr avr-libc python3-serial gawk \
          gcc-arm-none-eabi gcc-riscv64-unknown-elf libreadline-dev \
          picolibc-arm-none-eabi picolibc-riscv64-unknown-elf

Aside from make also being needed (rather obvious), I had to install the following on a fresh system as well:

• asciidoctor
• ruby-asciidoctor-pdf
• gcc-arm-linux-gnueabi