Clean up the benchlib compilation

[hugomg]

The way that benchlib uses 'make' to avoid recompiling benchmarks has bitten us multiple times in the past. We should either fix it so that the "smart recompilation" works properly, or we should replace it by something simpler and less finnicky.

Maybe there is some good inside the benchlib, but today my mood asks to throw it into the fires of Mount Doom. 🌋

[srijan-paul]

Does the CI cache the shared libraries generated by pallenec/cc or something?

[hugomg]

I think it shouldn't be caching anything between CI runs. See also #420.

[srijan-paul]

I see. I misunderstood the problem. So the caching is a problem when running locally.