@daviewales Hi. This is how I set up the Designer plugin in a `qscintilla2` formula update I did:

```ruby
    libexec.mkpath
    cd "designer-Qt4Qt5" do
      args << %Q["INCLUDEPATH+=#{include}"]
      inreplace "designer.pro" do |s|
        s.gsub! "$$[QT_INSTALL_LIBS]", lib
        s.gsub! "$$[QT_INSTALL_PLUGINS]", libexec
      end

      system "qmake", *args
      system "make"
      system "make", "install"
    end
    # symlink QT Designer plugin (note: not removed on formula uninstall)
      ln_sf libexec/"designer/libqscintillaplugin.dylib",
            "#{Formula.factory('qt').opt_prefix}/plugins/designer/"
```
I just tested under 10.9 and it is OK, but I bailed on submitting this a while back because it crashed Designer under 10.7.

Also note that I am passing `*args` to the install command. In my formula update, [the `args` are comprised of setting the `-spec` to `unsupported/macx-clang-libc++`](https://github.com/dakcarto/homebrew-osgeo4mac/blob/master/boneyard/qscintilla2.rb#L13-L19) for 10.9, then used for each C++ build. This ensures the Designer plugin is also built against libc++ on Mavericks.