help2man: restore nls support on darwin, make nls explicit

[thefloweringash]

Motivation for this change

When trying to use help2man in a package, I see the following error:

help2man -i ssh-agent-filter.help2man -o ssh-agent-filter.1 -N -L C.UTF-8 ./ssh-agent-filter
help2man: no locale support (Locale::gettext required)

The configure script for help2man uses gettext if available, so if gettext is not available it will silently build without nls support. If I pass --enable-nls to configure:

❯ nix-build -E 'let pkgs = import ./. {}; in pkgs.help2man.overrideAttrs (_:{ configureFlags = [ "--enable-nls" ]; })'
[...]
checking for library containing bindtextdomain... no
configure: error: bindtextdomain() required

I think this was caused by e73d334 in #142602. cc @Artturin

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[Artturin]

@ofborg build help2man