highlight: use $CXX instead of c++ to build

[illegalprime]

Motivation for this change

set $CXX and $AR so cross compilation works

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[Mic92]

@GrahamcOfBorg build highlight

[Mic92]

@illegalprime could you ask for ofborg access? NixOS/ofborg#331

[illegalprime]

@Mic92 thanks, will do!