rubyPackages.eventmachine: fix darwin build

This fixes the build of mailcatcher (and maybe others).

Fixes:
```
em.cpp:1232:40: error: invalid operands to binary expression ('__bind<int &, sockaddr *&, int &>' and 'int')
                if (bind (sd, bind_to, bind_to_size) < 0) {
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~
```
Where `bind` is treated as `std::bind` rather than `::bind` in newer C++
standards.

pkgs/development/ruby-modules/gem-config/default.nix

@@ -203,7 +203,12 @@ in
   };
 
   eventmachine = attrs: {
+    dontBuild = false;
     buildInputs = [ openssl ];
+    postPatch = ''
+      substituteInPlace ext/em.cpp \
+        --replace 'if (bind (' 'if (::bind ('
+    '';
   };
 
   exif = attrs: {