Makefile: build targets in parallel

I was never really happy with the way targets are serialized, it's fighting against `make` (causing issues like #234 and #219), and the target header isn't particularly informative. This fixes #234 and should also provide a speedup when building all targets simultaneously.

Download rules are now pre-requisites of build rules to stop multiple download attempts, and all build logs have a target suffix.

More work is needed on #219, since download rules are always outdated unless a marker file is used. I'll look at this separately as I also want to allow script only packages.