add a formula for compiling cryptopp with libstdc++

As described here http://blog.inventic.eu/2014/01/qt-on-osx-maverick-undefined-symbols-for-architecture-x86_64/ you need to compile cryptopp with libstdc++ to include it into a qt project
Homebrew's qt5 is also built against libc++ on 10.9.

Did you upgrade to 10.9 recently, by any chance? If you'd installed qt5 before upgrading it would have been linked against libstdc++. Or did you built qt5 with another compiler, like GCC?