Motivation for this change

I found that transfig doesn't build fig2dev on darwin, which is needed to build documentation for coq. This updates to the latest version of xfig, and splits out fig2dev into its own build. Since I'm not entirely sure whether this supplants the earlier build of transfig (especially because there is no longer a transfig binary as a result), I would like solicit comments on this PR.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-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/)
• Fits CONTRIBUTING.md.