Fixes compiler incompatibilities on 10.9

I was seeing the following error until I added this patch:

    Error: qwt60 dependency qt was built with the following
    C++ standard library: libc++ (from clang)

    This is incompatible with the standard library being used
    to build qwt60: libstdc++ (from clang)

    Please reinstall qt using a compatible compiler.
    hint: Check https://github.com/mxcl/homebrew/wiki/C++-Standard-Libraries