Motivation for this change

Since version 20200313 neomutt can compress network traffic when using imap (see the changelog). Build neomutt with zlib to enable support for this. It will be on by default and can then be toggled with imap_deflate.

Things done

Compiled with zlib as per this pr and verified that neomutt runs and 'echo $imap_deflate' yields yes (vs not set when using neomutt from current nixpkgs).
The closure size does not increase (zlib was already in the closure apparently).

• 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.