The makefile (counterintuitively) checks src/gen/customRuntime to see if
the bytecode is up to date. By simply touch(1)-ing it, we can save
maintaining a patch.
Many thanks for the various rewritings by @sternenseemann!
- The intermediary recompilation phases are hidden by default inside a
`lib.dontRecurseIntoAttrs`; only the zeroth and the second ones are exposed.
- `clangStdenv` is used by default in non-cross builds.
- The standalone build has an inferior priority.
- The mbqn sources are explicitly taken from mbqn instead of hardcoded in cbqn.
