Motivation for this change

Building openjfx11 or openjfx12 on a machine with many cores can consume huge amounts of RAM. With 24 cores, the memory usage exceeded 25GB per build for me.

Using NIX_BUILD_CORES to reduce the number of parallel tasks doesn't help, because a script invoking cmake does not respect NIX_BUILD_CORES and passes -j <number of cores>.

Setting NUMBER_OF_PROCESSORS overrides the automatic core count detection in the script.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.