Proposal: port build-bench-env.sh to make

[derSteFfi]

I recently had to rebuild the env a lot of times, sometimes partly (due to some components not building without errors). There is a lot of logic in that script that would be easily represented in Make logic:

• if $foo not present, download it
• if $dir not present, create it
• if $bin not present, compile it

and so on. Especially when not needing to regenerate certain parts of the build, make is just super effective and fast in skipping it. I'd write up a proposed Makefile that directly translates the script (as close as possible), but wanted to hear feedback first. What's the vibe for a Makefile replacing the script?

[jvoisin]

Sounds good to me, as long as the Makefile doesn't use obscure black-magic, it'll be an improvement over the current ghetto shell script.