acme-client fails to build with clang, which breaks the build on darwin.

Motivation for this change

Trying to fix broken builds on Darwin for ZHF.

ZHF: #97479

acme-client has a compilation error:

./compat.h:55:8: error: expected parameter declarator
size_t strlcat(char *, const char *, size_t);

This error goes away when compiling with gcc, so I set the package to use gccStdenv, after looking to see what similar packages did for this sort of issue.

If there's a better way to handle this, I'm open to feedback.

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.