libressl: work around some problem with man pages

https://github.com/NixOS/nixpkgs/commit/20ffc3cd73#commitcomment-22368612

pkgs/development/libraries/libressl/2.5.nix

@@ -13,6 +13,8 @@ stdenv.mkDerivation rec {
 
   outputs = [ "bin" "dev" "out" "man" ];
 
+  dontGzipMan = if stdenv.isDarwin then true else null; # not sure what's wrong
+
   meta = with stdenv.lib; {
     description = "Free TLS/SSL implementation";
     homepage    = "http://www.libressl.org";