Compilation issues on libstdc++10

[elshize]

It seems like there are some issues regarding PSTL when compiling against libstdc++10.

I'm not entirely sure what the problem is. It might be related to the fact that this version of the library has its own parallel algorithms implementation, but not sure. In general, PSTL has been moved to the LLVM project, so I'm not sure what we want to do with that in the long run.

I don't have any more info at the moment, I can add some error messages later, but I mostly wanted to point this out for later investigation, and for the reference for anyone who could have the same problem.

Note that I compiled it with clang11/libc++11 just fine, and this version I think actually has the standard parallel algorithms implemented using TBB & PSTL under the hood. So there is some logic in PSTL that considers this.

Again, the problem is that we can't really upgrade (easily?) PSTL because it seems to be now part of LLVM repository.