Add patch for clang 425 in C++11 mode.

See [this discussion](https://github.com/Homebrew/homebrew-science/pull/565) for some background on how we came across this. The patch was submitted [upstream](http://cgal-discuss.949826.n4.nabble.com/Compilation-fails-in-C-11-mode-with-Clang-425-on-OSX-td4658819.html). I'm not sure if there's a way to apply the patch conditionally on the compiler version, but it shouldn't hurt later versions of clang. Thanks to @sxprophet for this!