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
That's going to depend on what qt you use. Homebrew-installed qt uses libc++, so Homebrew's qt is compatible with the unmodified cryptopp formula.

Homebrew doesn't provide alternate packages like these to accommodate building against non-Homebrew libraries like qt. I'd suggest building cryptopp with a non-clang compiler, like gcc-4.2, if you're looking to make sure it uses libstdc++.

@Homebrew/owners I'm -1 on including this formula for the above reasons.