I saw this syntax in the [mutt formula](https://github.com/grimradical/homebrew/blob/master/mutt.rb), but I apparently didn't finish their implementation. Sorry that was bad form on my part; I'll fix it.

Regarding `-DTIXML_USE_STL`, tinyxml uses its own custom string type as default. You can use STL strings if you define `TIXML_USE_STL`. The debian maintainers decided that they want the shared library to use STL strings, so they made the [enforce-use-stl.patch](http://patch-tracker.debian.org/patch/series/dl/tinyxml/2.6.2-2/enforce-use-stl.patch) which make STL strings the default. The CMakeLists.txt patch also throws in `-DTIXML_USE_STL`, but it's not necessary if the enforce-use-stl.patch is used.