Get vim_configurable compiling on darwin

I had to do several things to get this working:

* add `libintlOrEmpty` to `buildInputs` for `atk`, `gdk-pixbuf` and `gtk2`
* fix include path for `pixman` in `cairo`
* fix include path for `cairo` in `pango`
* allow cups to be disabled in `gtk2` by setting a flag in `config`
* use `gccApple` to build `vim` (the configure script incorrectly determined the architecture on which it was building and defined some X11 function incorrectly)
* enabled `darwinSupport` when building on darwin
* disabled `patchelf` when building on darwin

As usual, I have tested this on my own OS X box and a CentOS box sitting on EC2.