Motivation for this change

• Remove which as build-time input
• Use substituteInPlace instead of sed (warns if it fails to replace)
• Use perl in buildInputs as it's a run-time dependency
• Patch shebangs manually, not sure why fixupPhase doesn't do it

Fixes #11563

You can test whether it's not completely broken with:

$ sudo mkdir -p /etc/ppp/peers
$ sudo ./result/bin/pptpsetup --create TUNNEL --server SERVER --username USER --password PASSWOWRD

$ ./result/bin/pptp localhost
anon warn[pptp_gre_bind:pptp_gre.c:102]: socket: Operation not permitted
anon fatal[main:pptp.c:360]: Cannot bind GRE socket, aborting.

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 nix-review --run "nix-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.