Save memory. Do not link to LLVM libraries in parallel

[ellert]

This Pull request:

Linking to the built-in static LLVM libraries uses a lot of memory. Linking more than one such target in parallel can exhaust the memory. This PR adds dependencies to prevent this.

Compare

https://github.com/root-project/root/blob/a32e7d0eb6ff583c3420e32b4198d20cd7e8ebd4/core/metacling/src/CMakeLists.txt#L126-L127

[dpiparo]

Thanks for these changes. How much memory is consumed in the worst case? Is some potential for parallelism lost at the risk of some potential reduction of build time?

[hahnjo]

These dependency hacks are not really great for targeted rebuilds; for example the current hack already results in ninja Cling (which I do quite often) rebuilding also rootcling_stage1. If it's possible to use ninja, we could look into JOB_POOLS in the way that LLVM provides LLVM_PARALLEL_LINK_JOBS.