zmq: add enableDrafts option

This is needed by emacs-zmq, and will be used by it.

pkgs/development/libraries/zeromq/4.x.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchFromGitHub, cmake, asciidoc }:
+{ stdenv, fetchFromGitHub, cmake, asciidoc, enableDrafts ? false }:
 
 stdenv.mkDerivation rec {
   name = "zeromq-${version}";
@@ -17,6 +17,8 @@ stdenv.mkDerivation rec {
 
   doCheck = false; # fails all the tests (ctest)
 
+  cmakeFlags = if enableDrafts then [ "-DENABLE_DRAFTS=ON" ] else null;
+
   meta = with stdenv.lib; {
     branch = "4";
     homepage = http://www.zeromq.org;