Ninja build system

I made a start on using Ninja to simplify the build system and fix #123 (closed). Currently it only generates a ninja.build for all the variants of the main body. You can try it out with:

pip3 install ninja
python3 generate_build.py # generates the build.ninja file
ninja # uses the build.ninja file to run the build

We could consider condensing the last two commands to just ./build.py and regenerate the build.ninja on every build (as it doesn't take long).

@rwb27 please have a look at the generate_build.py to see if you like this approach. An alternative to this that I found is OSBS which is worth having a look at, but would require adapting our scad files to use its conventions.